iButton Electronic Lock
Since iButton DS1990A introduced
in market from Dallas Semiconductor (MAXIM), it has been used in many applications concerning
security, access control systems etc. In this project we will use iButton as a
key to an electronic lock. This electronic lock can use many different kinds of
iButtons and can store up to 9 different keys. One of the keys is the master
key and is permanent stored in memory. With the use of master key we can add or
remove slave keys.
This electronic
lock can be used with any type of iButtons you may already have, since the only
thing needed is the internal serial number, that’s different for every iButton.
The command used to read the serial number is the same for all iButtons. The
iButton family code that goes with every iButton, can be anything and is
calculated as part of the whole serial number. We must also notice that DS1990A
series iButtons are the cheapest.
This electronic
lock designed to work stand-alone and it’s easy to construct. What the user
sees (outside of the door for example) is a iButton socket and a led. From
inside the door, we can open it using a simple push button. For the actual lock
of the door a solenoid and a bold are used. Solenoid must be rated at 12Vdc.
iButtons serial numbers stored in memory can be removed and updated when
needed. One master key is used to manage the rest of them. Totally a number of
9 different keys can be stored in memory.
Schematic diagram is
shown at figure 1. The circuit is build around an Atmel AT89C2051(U1)
microcontroller. The port 1 (P1) of mcu is used to connect a 7-segment common
anode led display. This led display will be used on the programming of
additional keys. For the same reason a push-button labelled SB1 is connected on
P.3.7. Storage of iButtons serial numbers is done on a 24C02 EEPROM (U3). It is
connected on P3.4 (SDA) and P3.5 (SCL) of U1. The external iButton socked is
connected on port P3.3 via XP2 pin array. The rest of components VD4, R3, VD5 and VD6 are used for protection
of mcu ports. One pull-up resistor R4 is used as required from 1-wire protocol.
An additional iButton socket is connected parallel with the predefined at pins
XS1. This one is used for programming the keys. The door OPEN button is
connected on P3.2 through XP1 connector, using the same protection components
as above. The solenoid that does the lock is connected on XT1 connector.
Solenoid is controlled from a power MOSFET IRF540 (VT3). Diode VD7 is added to
protect MOSFET from voltage strikes due to solenoid inductance. Transistor VT3
is controlled from VT2, which reverses the logic state that’s appears on P3.0,
so on VT3 we have output 0V and 12V. This additional transistor is useful as it
translates the mcu logic levels to 0V and 12V, capable to drive the solenoid.
Fig.1 Schematic diagram of iButton electronic lock
A led is used to indicate the state of the electronic
lock, which is controlled from the same pin as the solenoid, using transistor
TV1. This led is connected to the board using the same pin array XP2. But we
need to ensure that the circuit will always work without supervision. For that
reason we added ADM1232 (U2) that does the mcu reset pin control. This chip
have a counter and voltage test circuits inside it. On pin P3.1 mcu produces
pulses when it works right. If for a reason mcu freeze then U2 send it a reset
pulse and work is resumed.
This electronic lock has it’s own power supply on board, consisting of
transformer T1, bridge rectifier VD9-VD12 and voltage regulator U4. As power
backup an array of 10 AA batteries is used (BT1-BT10). Total capacity is 800mAH. When the circuit is connected on
main voltage the battery pack is charged via R10 with a current of 20mA. This
current is equal to 0.025C (where C is the batteries capacity) and that’s a
very small current depending on total capacity. That’s put the battery on a
steady charge to compensate losses among time and no charge completion
detection is needed. That can be done as the excess energy is consumed in heat,
that can not harm batteries as its low.
Overall board dimensions are 150х100х60mm. The most components are placed on the board, including the
transformer. Batteries are placed on battery holders. In the place of AA
batteries we could use a 12V sealed Lead – Acid battery. External components
are connected on board with 2 or 3 pin connectors. Part numbers HG1, SB1 and XS1 are used only in programming
mode so can be placed inside the plastic enclosure. Led VD3 can be placed on
the face of enclosure, to indicate
proper powering of board. A connection diagram is show on figure 2.
Fig.2 Connection diagram
When the door goes open, a 3 sec pulse is triggering the solenoid. When
we press the door open button the door remains open as long as we push it.
The electronic lock can register 9 keys, plus one master key. Master’s
serial number is stored inside mcu. The rest of keys are stored on the external
memory under slot 1 to 9. To add or remove a new key you should have the master
key. Also master key can be used to open the door.
Fig. 3 Programming steps for adding a new key
To add a new key, the following steps should followed:
- Press programming button.
- Led displays letter «P» that indicated
you entered programming mode.
- Touch the master button in socket.
- Led displays number «1». That’s the current
selected slot in memory.
- Push the programming button to select a different
programming slot for your new key.
- Touch the new key to the socket.
- The number on led display blinks, indicating
ready to program.
- Touch again the new key to confirm registration
to memory.
- If successfully registered the display stops
blinking.
- After 5 seconds, the program exits from
programming mode.
The programming
procedure to register a new key is displayed schematically on figure 3.
If you want to register more keys, then from step 9 you can go directly
to step 5. These steps can be revised as many times as you like.
If after step 7, you find out that you selected wrong slot number and
you don’t want to loose that key, press programming button or just wait 5
seconds. When you press the button the slot number increases by one and memory
hasn’t changed yet. If you wait 5 seconds, programming mode will exit and
nothing is going to register in memory.
Generally in any programming step, you can wait 5 second to exit programming
mode.
To remove an already registered key, you follow an almost same
procedure, using only the master key. Basically, it’s like registering the
master key on the memory slot you would like to erase. This procedure is shown
on figure 4.
Fig.4 Programming steps for removing a key.
During programming
mode, the door will only open with the press of the OPEN button. Also, because
the two iButton sockets are connected in parallel, you should avoid simultaneous
touch of keys on both sockets.
Master’s key serial number is stored on mcu’s program memory, beginning
from address 2FDH. The length of serial number is 8 bytes. The serial must be
equal that is printed on top the iButton case, reading from left to right. On memory address 2FDH the control byte is
registered, then on address 2FEH - 303H the
next six bytes are registered, beginning with most significant byte. Finally
the family code byte is stored on address 304H. For example a complete serial
code should look like: 67 00 00 02 D6 85 26 01
The software block diagram shows on figure 5. The
program starts, asking if a key has entered. If a key is entered, then it goes
on reading the internal serial number. The next step is to check if this is the
master key or another key already registered in memory. If the key is verified
then the door is opened. Also the OPEN push button is checked, and if it’s
pressed the door opens.
Fig.5 Software block diagram
For the programming mode there exists two subprograms: PROGT and PROGS, whose block diagrams
show in figure 6. The first is called when the serial number is read, in the
programming phase and the second is called when the programming button is
pressed. Programming of a new key is completed in three phases. When we press the programming button, we enter the programming mode. In this state
the led displays «P» and the serial number of the
key is checked to see if this is the master key, because this key is required
to proceed on programming steps.
If this is the master key, we proceed on phase 2. Now, led is displaying
the number of current selected memory slot, changing by pressing programming
button. If we touch the key again, then it is registering on memory and we pass
to phase 3. If we touch another key, this is also registered and we pass to phase
2. With the press of the button, we pass on phase 2 without registering any
key.
If we don’t touch
anything in a period of 5 seconds, the program exits from programming mode.
Block diagrams of figure 5 and 6 are simplified, but they give as an
overall sense of program functionality.
It’s upon your
desire to extend the capabilities of this program, as it’s open source, to fit
your special needs.
Fig.6 Programming mode
subprograms block diagrams
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