eLab Hackerspace GSM Access Control System

door handle mechanism 4.JPG

Hi! 🙂

As you may already know, I am one of the founders of eLab Hackerspace, in Faro, Portugal. In case you didn’t read it, I have made a post about the birth of eLab Hackerspace before. It is located inside the Institute of Engineering of the University of the Algarve and its members are mainly electronics and informatics students, from BSc to PhD. However, as usual in most hackerspaces, everyone is welcome at the space 🙂 . Since it is located inside the campus, we have some benefits as not paying rent, internet, electricity or water. In order to allow all members to enter the hackerspace at any time and keep a register of who accessed and when, we decided to build an access control system. The log of entrances may be useful in case anything gets damaged, stolen, or even just to give some prize to the members that spend more time there 😛 Check the video below to see the developed access control system in action! 🙂

We always try to reuse old equipment in our projects, in order to save money and help protecting the environment by extending the lifetime of some devices. Also, we didn’t have much money to spend at the time we started building it. Actually, the financial situation hasn’t changed since we’re keeping the membership low (2€ per month) to get more people involved. Having that said, we looked in our “trash” boxes to see what we had that could be used to make an access control system for the hackerspace. We could have just left a computer on running a webserver that we could access via Wi-Fi with our phones, but not everyone has a smartphone. We could also do the same kind of thing with Bluetooth, but once again, not everyone has a cell phone with Bluetooth. Moreover, that would be too simple! Where’s the fun in doing things so simple? Isn’t much more fun to overengineer something? What about a full McGyver challenge? 😛 So, after diving into our “trash” boxes we had something we could work with: a working Siemens C55, a working old computer, one good old ATX power supply, and a DC motor with gearbox removed from a tobacco vending machine.

access control system 1.JPG

Taking into account the parts we had, we decided to do a GSM Access Control System! The idea we had was to make a system that would allow anyone to open the hackerspace door just by ringing a phone number! How cool would that be? Anyone would even be able to open the door with an old mythical Nokia 3310! 😛 And we could also connect the system to a computer that would keep a log of the entries and sending it to the web so that we could see which members were at the hackerspace in real time! 🙂 With the project idea defined, it was time to start working on it. Since other eLab Hackerspace members were busy with other projects, it was only me and Gonçalo Pereira developing the access control system. I’ll now describe every part of the project, step by step.

1 – Door opening mechanism

door handle mechanism 2.JPG

The door of the hackerspace is an aluminium door with an emergency exit lever. We weren’t allowed to drill or modify the door in any way, so we tried to find a way of using the lever to open the door. Gonçalo engineered a way of using the geared DC motor to pull it to open the door and also found a way of attaching the motor to the door without any drilling or glueing. He attached the motor to a piece of metal and then used the silicone frame of the aluminium door to hold it in place. It turned out to be surprisingly easy to put the motor in place. To pull the lever, he placed an off-centered pin in a gear that was already attached to the motor’s axle and then used a small piece of a pulley taken from an old printer to attach the off-centered pin to the lever. It’s basically the same type of mechanism used in car pistons.


He also added a switch that would be triggered by the rotating gear when the lever was in the “open” position. After the mechanics of the system were done, he designed a simple board with a PIC16F88 microcontroller (more details about the board on section 4) based on the PIC16F88 prototyping board that I had previously designed for us to use at the hackerspace. He also did some simple programming of the PIC16F88 just to test the door opening mechanism: when a button was clicked, it would turn on the motor until the “open” switch was activated, then wait three seconds, and then turn on the motor again until the “open” switch stopped being activated, which meant that the lever was in the “closed” position. However, we ran into our first problem: the motor was so noisy that it was generating voltage spikes in the pin that was supposed to read the switch. We tried putting the noise filtering capacitors in the motor’s pins, tried using a more stable power supply, but we weren’t being able to filter it using hardware. After some time trying to fix the problem using hardware, I remembered a simple and clever way that worked like a charm: implement a software mechanism that would ignore isolated voltage spikes. I simply implemented a counter and defined a counting threshold: the PIC16F88 would count every time that the pin read a logic 1 but would only consider that button as being activated when the threshold value was reached, The counter would increase its value with the voltage spikes, but when the switch was actually activated, the counting value would increase extremely fast, reaching the threshold value. Having the opening/closing mechanism ready, it was time to start working on the GSM part of the system.

2 – Siemens C55 working as a GSM modem

In case you didn’t know, some old cell phones such as the ones from Siemens, Nokia and Samsung are really good for electronics projects. Some of them can be used as GSM modems, having a simple serial connection and a communication protocol based on AT Commands. The amount of supported commands may vary, depending on the model of the phone and on the manufacturer. The Siemens C55 turns out to be one of the best phones for electronics projects since its GSM modem supports a wide list of AT commands.

access control system siemens c55 2.JPG

The first thing we needed was a SIM card. Luckily we had a SIM card from Vodafone that was given to a member of eLab in some promotion. The card had a prepaid plan with no mandatory payments. The only obligation to keep the number active is to make a phone call every 6 months. The card also came with a bonus of 5€, which should last forever making only a 1 second phone call twice a year 🙂 The second thing I needed was a serial data cable for the Siemens C55, which I didn’t had :\ I tried to find a cheap cable on ebay but I was out of luck. However, I had two Siemens chargers with the correct connector but only with the VCC and GND pins. I had no option but to hack one of the connectors to add the Tx and Rx pins and it worked just fine! 🙂 Once I had the connector ready, I searched online to find which was the default baud rate for the Siemens C55 and I found some websites saying that it was 9600bps, and others saying 38400bps. After searching for the baud rates, I connected the Tx and Rx pins to the Rx and Tx wires of a USB to Serial LVTTL cable and used putty to establish a serial connection with the phone. I decided to try with 9600 first. Once the connection was open, I typed “AT” and saw the letters being echoed to the putty terminal, which was a sign that the phone’s GSM modem was replying and using a 9600 bps baud rate 🙂 After clicking “Enter” the phone replied “OK” which, once more, was a good sign that the GSM modem was replying to the AT commands. Then I used the command “AT+CPIN=xxxx” , where <xxxx> is the pin of the SIM card, followed by “Enter” and once again the modem replied with “OK”. 🙂

When someone makes a phone call to the phone number of the card inserted in the GSM modem of the phone, it usually only replies with “RING” every time you hear the calling tone. In order to activate the identification of the calling numbers, I had to send one more command: “AT+CLIP=1”. After issuing this command and making a new phone call, the phone finally replied with:


+CLIP: "911234567",161,,,,0

I had now the confirmation that the phone could provide all the data that I needed to continue the development of the access control system. 🙂

By the way, if you try to send AT commands to a phone, it may happen that it won’t echo the characters that you send it. Some phones come with the echo disabled. It may happen that you type “AT” followed by “Enter” and only receive the response “OK”. To activate the echo, you can send the command “ATE1”. To deactivate it, you can send the command “ATE0”. If you wish to know more about AT commands for the Siemens C55, see this document.

3 – RS232, TTL and LVTTL serial communications board

rs232 lvttl ttl communications board.JPG

The next step was to establish a serial connection between the PIC16F88 board, the Siemens C55 Cell Phone and an old computer via RS232 connection. Once again I faced some problems: the PIC16F88 was supplied with 5V and was working with TTL levels, the Siemens C55 phone required LVTTL levels and was not tolerant to TTL (it would turn itself off), and I still had to establish a connection with the computer using RS232 levels. Moreover, I had do create a UART communication with one Master (the PIC16F88 board) and two slaves (the Siemens C55 and the computer) which is something that requires a little trick to work. This trick consists in placing a diode in the Tx line of every Slave device, with the Cathode facing the device. Why is this necessary? Well, according to the UART specifications, when a device is in the inactive state (idle), its Tx pin must be high. When communication occurs, it is the Slave’s responsibility to pull the Tx line down to sink the Master Rx channel’s current. Using the diodes, when in idle state, every Slave is able to keep his own Tx high irrespective of what the master’s Rx channel is at. A single Slave can transmit by pulling his Tx line down without affecting the Tx on other slaves. Having the Master/Slave trick done, it was time to do some level conversion. To convert the 5V TTL signal from the Tx pin of the PIC16F88 to an LVTTL level, a voltage divider using two resistors was used. To convert the 3.3V LVTTL signal from the Siemens C55 Tx pin to 5V TTL, an MCP6002 Op Amp was used as a comparator, pulling the voltage level to 5V everytime the Tx signal from the cell phone passed the 2.5V threshold value, defined by a voltage divider built with two resistors. The conversion between TTL and RS232 levels was a lot simpler and was done by using a single MAX232 chip. Below you can see the PCB and the schematics where you can identify all the parts that have just been described.



The board was designed in Kicad. You can download the PCB files by clicking here.

4 – PIC16F88 board

pic16f88 control board 2.JPG

As referred, the board was designed by Gonçalo Pereira. It is supposed to by powered with 12V and contains a 5V linear regulator do power the PIC microcontroller. It also contains a channel N MOSFET acting as a switch controlled by the MCU to turn the motor on and off. Besides that, it also contains all the necessary connectors for power, motor, motor switch and UART communications. The board was designed in Eagle and you can download the Eagle project files here. The firmware for the PIC16F88 is discussed and can be downloaded in the next section. Below you can see the schematic and PCB of the board.



5 – PIC16F88 programming

All the code was written in C using Piklab and compiled with SDCC (see here how to setup a microcontroller development environment with Piklab and SDCC). You should be able to compile it with other compilers with some minor modifications. You can also use the .hex file included in case you don’t want to change anything. When developing the code we decided that the member’s phone numbers would be stored in the PIC’s EEPROM. This way, if the Internet connection failed for some reason, the door would still open for an authorized member, although the entrance would not be registered in the entrance log. The PIC16F88 has 256 bytes of EEPROM. Taking into account that all the portuguese cell phone numbers have 9 digits and the first one is always a 9, we had to store 8 digits for every phone number, which allows us to store 32 phone numbers in the PIC’s EEPROM. It may seem that 32 is a small number but eLab Hackerspace is also a small Hackerspace and has only a few members. If the day comes when we get more than 32 members, we’ll be happy to replace the PIC with another one or even replace most of the Access Control System with a Raspberry Pi or something else 😛 Having the data storage defined, it was time to create an AT command protocol that would allow us to add, delete and view phone numbers stored in the internal EEPROM. Every command starts with “ELAB+”.Here’s a list of commands that were implemented:

  • ELAB+P=<PASSWORD> – Insert admin password to enter admin mode. The <PASSWORD> field is to be replace with the real password, hard coded in the PIC’s firmware. In case the password is accepted, the PIC emits a “AUTH OK” response. Otherwise, it replies with “NO AUTH”.
  • ELAB+A#XX=<NUMBER> – Add a number to position XX (from 01 to 32) where the <NUMBER> field must be replaced with the 9-digit phone number. This command only works after the admin mode has been activated. Otherwise you’ll receive a message saying “NO AUTH”. If the command is accepted you should be replied with an “OK”. In case the command is not correctly entered the PIC will reply with “ERROR”;
  • ELAB+D#XX – Delete the number located in position XX (from 01 to 32). This command only works after the admin mode has been activated. Otherwise you’ll receive a message saying “NO AUTH”. If the command is accepted you should be replied with an “OK”. In case the command is not correctly entered the PIC will reply with “ERROR”;
  • ELAB+V – View the full list of numbers stored in the internal EEPROM. This command only works after the admin mode has been activated. Otherwise you’ll receive a message saying “NO AUTH”. If the command is accepted you should be replied with an “OK”. In case the command is not correctly entered the PIC will reply with “ERROR”.

The default password is located in the password.txt file contained in the “PIC16F88 Control Board Firmware.zip” file. You can change it in the source code and recompile it if you want to change it.

To use the AT command interface, the system administrator just has to open the serial connection (9600bps baud rate) with the PIC microcontroller and send the commands above to perform add, delete and view operations. When all operations are made, the Reset button on the PIC board should be pressed to reactivate the security lock in the PIC microcontroller. A lot of improvement can still be done in the code, such as adding a command to reactivate the security lock or adding a command to open the door, among other possibilities.

Whenever the PIC receives a CLIP message from the cell phone, it checks if the received number exists in its internal EEPROM. In case it exists, it sends a “A:<9_DIGIT_NUMBER>” to the computer. In case the number does not exist, it sends a “R:<9_DIGIT_NUMBER>” to the computer.

Regarding the door opening mechanism, once a message is received with a valid number, the motor is activated until a switch located near the motor is activated T times (to ignore the voltage spikes caused by motor noise). Then it waits 3 seconds and activates the motor again until the switch is released.

You can download the firmware by clicking here.

6 – Python application to send data to Google spreadsheets

As referred, in order to create an online entrance Log and to create a public log of daily entrances, we used an old computer to send the data to a spreadsheet on Google Docs. Of course we would prefer using a Raspberry Pi for this purpose, but we couldn’t find nor a Raspberry Pi in our trash boxes nor 40€ to buy one 😛 Moreover, for proof of concept an old computer works perfectly fine. The computer is running a lightweight Linux distro: Lubuntu 12.04. All the code to send data to the spreadsheet was developed in Python and I advise that at the moment the code is not pretty and not commented, since it was written quickly just to get the system working. In order to be able to send and receive data from a Google spreadsheets we first had to install the Google Data API. To install it, just go here, download the latest version, extract the files into a folder and then run the script setup.py inside the folder by entering the following command in the console:

./setup.py install

Before beginning coding, we had to set up two Google Spreadsheet files: one to store the list of eLab members, with columns for their names, cell phone numbers and some other contact info; and another one to be the eLab Entrance Log, containing columns for the date, hours, phone number, name of member, and if the entry was accepted or rejected. The second spreadsheet file contains also two other spreadsheets, being one of them just to perform operations on data for visualization and the second one being used to be the daily entrance log, with the entries of the current day being fetched from the first spreadsheet. Here you can see and download sample spreadsheets:

For serial port communications we also had to install pyserial using the following command:

sudo apt-get install python-serial

Once we had the system working I thought it would be fun if every member could have its own entrance music 🙂 So, I installed pygame using the command

sudo apt-get instal python-pygame

and added a new column for the song filename in the eLab members spreadsheet and also added a few more lines of code to the Python application to play the song. Of course at the beginning I had to joke around and put some stupid music for some eLab members entrances 😛 As an example, now when I enter it plays the epic Pirates of the Caribbean theme but I’ll have to change it soon since I (and everyone else!) have had enough of it 😛

Ill now describe what the Python source code does, in a general way. Whenever the Python application receives a “A:<9_DIGIT_NUMBER>” message from the PIC16F88, it fetches the data from the Google spreadsheet containing the eLab Hackerspace members information and checks which user has the received phone number. After checking which user has the received phone number, it plays the corresponding entrance music and it sends the entry information to the eLab Hackerspace entrance Log spreadsheet. In case it receives a “R:<9_DIGIT_NUMBER>”, it sounds the alarm sound and sends the rejected entry information to the eLab Hackerspace entrance Log spreadsheet. Below you can see the code for the Python application. If you try to use it don’t forget to change the email, password, spreadsheet key and title of the members list spreadsheet. The spreadsheet key can be found on the url of the spreadsheet after the string “key=”.

import time
import gdata.spreadsheet.service
import pygame
import serial


email = 'user@gmail.com'
password = 'password'

auth = 'granted'
number = '913564168'

ser = serial.Serial("/dev/ttyS0",9600)

var = 'X'
action = 'X'
phonenumber = 'X'


spreadsheet_key = 'XXX'
# HAS A VALUE OF '0d6'
worksheet_id = 'od6'

spr_client = gdata.spreadsheet.service.SpreadsheetsService()
spr_client.email = email
spr_client.password = password
spr_client.source = 'Example Spreadsheet Writing Application'


#Query Members list
gd_client = gdata.spreadsheet.service.SpreadsheetsService()
gd_client.email = email
gd_client.password = password
gd_client.source = spr_client.source

	var = ser.read()
	pnumber = 0
	if var == "A" or var == "R":
		if var == "A":
			auth = "granted"
			auth = "rejected"
		var = ser.read()
		if var == ":":
			pnumber = ser.read(9)
			number = str(pnumber)
			print auth
			print number
			q = gdata.spreadsheet.service.DocumentQuery()
			#Below relace with the name of the member list spreadsheet
			q['title'] = 'eLab Members'
			q['title-exact'] = 'true'
			feed = gd_client.GetSpreadsheetsFeed(query=q)
			spreadsheet_id = feed.entry[0].id.text.rsplit('/',1)[1]
			feed = gd_client.GetWorksheetsFeed(spreadsheet_id)
			worksheet_id = feed.entry[0].id.text.rsplit('/',1)[1]

			rows = gd_client.GetListFeed(spreadsheet_id, worksheet_id).entry

			telefone = ''
			som = ''
			nome = "Desconhecido"
			done = False
			for row in rows:
				if done == True:
					done = False
			#	print " %s %s" % (row.custom['telefone'], row.custom['nome'].text)
				for key in row.custom:
					if key == 'nome':
						nome = row.custom[key].text
						print nome
					if key == 'som':
						som = row.custom[key].text
						print som
					if key == 'telefone':
						telefone = row.custom[key].text
						print telefone
				if telefone == number or nome == "Desconhecido":
					# Prepare the dictionary to write
					dict = {}
					dict['date'] = time.strftime('%d/%m/%Y')
					dict['time'] = time.strftime('%H:%M:%S')
					dict['number'] = number
					dict['auth'] = auth
					dict['nome'] = nome
					print dict

					entry = spr_client.InsertRow(dict, spreadsheet_key, worksheet_id)
					if isinstance(entry, gdata.spreadsheet.SpreadsheetsList):
						print "Insert row succeeded."
						print "Insert row failed."
					if not isinstance(som, basestring):
					print "Playing song!"
					done = True
					while True:
						#for event in pygame.event:
							while pygame.mixer.music.get_busy(): 
					print "End of song!"

To publish the daily Entrance log in a webpage you just have to open the spreadsheet on Google Drive, click on File -> Publish to Web  and then choose which sheet you want to publish and click the button to start publishing. Then, in the bottom of the popup window choose HTML to embed in a webpage and copy the HTML code and paste it in your webpage.

Here are a few more pictures of the system:

Response code is 404

And I think that’s everything you need to know to make a similar access control system for you hackerspace/garage/house/bedroom/etc. 🙂 In case I forgot to mention something you find necessary, just let me know in the comments section below! 🙂

I hope you liked this project. There is still a lot of room for improvement either in software or hardware.

This project is also available on instructables.comeLab Hackerspace GSM Access Control System

See you next time! 🙂

Update (05/12/13): This project has been featured on Instructables! 🙂 Check it out here.

Update (12/12/13): This project has been featured on Hack a Day! 🙂 check it out here.

Did you find this post helpful? Do you wish to contribute to other projects regarding computer science, electronics, robotics or mechatronics that will be posted in this blog? If you wish to do so, you can donate via paypal using the button below. Thanks! 🙂