Code locks can be constructed using digital Ics and timers based on valid inputs and invalid Key strokes. These circuits require too many Ics.

Here is a simple design based on ATMEL 89c2051 a 20-pin micro controller that does the whole job of detecting a 4-digit sequential code with time limit facility with the help of only one more additional Ic ( a buffer 4050) and a transistor to latch a relay.

With the cost of micro controllers now dropping as low as 4 digital IC gates ,

It makes sense to design simple logic circuits using them with free ware software languages.

The project uses an LCD display for design and development but it is not really necessary and can be removed from the circuit without any change in source code.

LCD  models are available in 16 pin or 14 pin configuration. The 16 pin variety has a back light option. Popular brands are LAMPEX, HANTRONIX and Hitachi. Most other models also have the same pin configuration.

Note the pin numbers before soldering to the circuit.

The model used n this project is LAMPEX –model LM16200 Alpha numeric with 16 character and 2 lines with back light option.

During our experiments , we found that any unbranded LCD display works as well and the project is not specific to any brand

The 10k Potentiometer controls the contrast of the LCD panel works best when it is nearer to ground potential.

The 2 line x 16 character LCD modules are available from a wide range of  manufacturers and should all be compatible.

The source program in BASCOM-51 is listed below:

'--------------------------------------------------------------
' file: efy20LOC.BAS
' micro controller based 4 digit code lock
' written in bascom-51 language from www.mcselec.com
' by K.S.Sankar www.mostek.biz june 2006
'--------------------------------------------------------------
' pass = 1 4 2 8 binary ( 1324) decimal

$crystal = 6000000
$regfile = "89c2051.dat"

Dim I As Byte
Dim K(4) As Byte
Dim Pass(4) As Byte
Dim Key As Byte
Dim Invalid_pass As Bit
Dim Sec_count As Byte
Dim Clock_word As Word
Dim Passtime As Byte
Dim Attempts As Byte
Dim Maxattempts As Byte

Ready_led Alias P1.0
Relay_out Alias P1.1

For I = 1 To 4
K(i) = 0
Next I

Pass(1) = 1
Pass(2) = 3
Pass(3) = 2
Pass(4) = 4

Sec_count = 0
Passtime = 5
Attempts = 0
Maxattempts = 3

Config Lcd = 16 * 2
Config Lcdpin = Pin , Db4 = P1.4 , Db5 = P1.5 , Db6 = P1.6 , Db7 = P1.7 , E = P1.3 , Rs = P1.2
'port 1

P1 = 0
P3 = 255

Config Timer0 = Timer , Gate = Internal , Mode = 2
'Timer0 use timer 0
'Gate = Internal no external interrupt
'Mode = 2 8 bit auto reload

' set t0 internal interrupt 2000 times a sec
On Timer0 Timer_0_overflow_int
Load Timer0 , 250
Priority Set Timer0
Enable Interrupts
Enable Timer0


Begin:
If Attempts >= Maxattempts Then
 Locate 0 , 0 : Lcd Maxattempts ; " attempts over"
 Locate 2 , 0 : Lcd "try after 10 seconds"
 Attempts = 0
 Gosub Trylater
End If

Sec_count = 0

For I = 1 To 4
 K(i) = 0
Next I


Cls
Cursor On Blink
'clear the LCD display
Lcd "Enter Pass:"
'display this at the top line

Ready_led = 1

For I = 1 To 4

 While 1 = 1
 If Sec_count > Passtime Then
 Exit For
End If

If P3 <> 255 Then
 ' some key pressed - check it

If I = 1 Then
 ' start timer0 on first keystroke
 Sec_count = 0
 Start Timer0
End If

 Key = P3
 ' wait for key release
 While Key = P3
 Wend
 K(i) = 255 - Key
 If K(i) = 1 Then
 Goto Lcd_out
 End If
 If K(i) = 2 Then
 Goto Lcd_out
  End If
 If K(i) = 4 Then
 K(i) = 3
 Goto Lcd_out
 End If
 If K(i) = 8 Then
 K(i) = 4
 Goto Lcd_out
 End If
 If K(i) = 16 Then
 K(i) = 5
 Goto Lcd_out
 End If
 If K(i) = 32 Then
 K(i) = 6
 Goto Lcd_out
 End If
 If K(i) = 128 Then
 K(i) = 8
 Goto Lcd_out
 End If

 ' invalid key combination
 Key(i) = 0

Lcd_out:

 Lcd K(i)
 Waitms 30
 Exit While
End If

Wend


Next I

Ready_led = 0

Stop Timer0

' check if time over
If Sec_count > Passtime Then
 Locate 2 , 0 : Lcd "time over"
 Incr Attempts
 Gosub Error_flash
 Wait 1
 Goto Begin
End If

' check valdity
Invalid_pass = 0
For I = 1 To 4
 If K(i) <> Pass(i) Then
 Invalid_pass = 1
 End If
Next I


If Invalid_pass = 1 Then
 Goto Invalid
End If



Valid:
 Locate 2 , 0 : Lcd "valid password"
 Relay_out = 1
 Wait 1
 Relay_out = 0
 Goto Begin


Invalid:
 Locate 2 , 0 : Lcd "invalid"
 Gosub Error_flash
 Incr Attempts
 Wait 1
 Goto Begin


Trylater:
' wait for 10 seconds
 For I = 1 To 10
 Wait 1
 Key = P3
 Key = 255 - Key
 If Key = 3 Then
 Exit For
 End If
Next I

Wait 2
Return

Error_flash:
For I = 1 To 10
 Ready_led = Ready_led Xor 1
 Waitms 100
Next I
 Ready_led = 0
Return




' interrupt subroutine -----------------
Timer_0_overflow_int:
' program comes here 2000 times a sec with a 6mhz xtal
Incr Clock_word

If Clock_word > 2000 Then
 Clock_word = 0
 Incr Sec_count
End If
Return

End
' ==-=-=-=-=-=-=-=-=-=-=-=-==-==

The program in hex is only 1.6k and the 2051 micro can take upto 2k of code.

This program can be modified to suits users requirement.

The hex file should be ‘burnt’ into the chip using any universal programmer.

Circuit diagram below:

Circuit Explanation

The micro controller used is ATMEL AT89c2051 a 20pin device with 2k of program memory.

Port-1 is used to drive the LCD display in 4-bit mode with 10k pullup resistors. A crystal of 6Mhz is used.  

The software switches on a ready led and waits for a 4 digit code. The valid code in this project is 1324.

The 8 input switches are connected to port-3 and the LCD to port-1.

Port-3 does not have the bit p3.6 and it is ignored.

Two leds are at port 1 bit0 and bit 1 to indicate a READY and  RELAY ON indication.

The relay ON pin is also connected to a transistor through a buffer to swtich ON a 12v Relay which can activate the electric lock.

A timer is started on the first key stroke and the remaining 3 digits have to be entered within 5 seconds. If not the software gets back to the beginning.

After 3 attempts the circuit will wait for about 10 seconds to avoid unwanted tampering attempts.

All these timings can be changed to suit the readers requirement in the source program.

The software is written using BASCOM-51 ( more about it in the authors previous article in JAN 2005 issue – REAL TIME CLOCK using micro controller)

The source code is self explanatory