Home > Circuits > UA3730 Combination Lock
Modified:20:36, 22 October 2013
This circuit, based on the UA3730 IC with some modifiactions to produce a combination lock for a safe. The IC has many features which you can utilise but just a basic keypad and solenoid has been used for this design.

The circuit has been adapted from the datasheet to only use the ON/OFF output2 (pin16) which goes low when the correct sequence has been entered. This is inverted by Q1 and the relay is then driven on by Q2. A standard PCB mount relay with a rating of 5A contacts should be sufficient.

The circuit will operate from 4.5 to 6 volts - the relay coil should be rated at 5 or 6 volts.

Diodes D2 to D4 are 1n4148 general purpose signal diodes.



The keypad should be wired up as shown left. The keypad can be replaced with push-to-make switches if desired as long as they follow the correct connections.

A suggested keypad is shown right. The pin connections are:

1 = Col2
2 = Row1
3 = Col1
4 = Row4
5 = Col3
6 = Row3
7 = Row2

There is a datasheet avaiable of the UA3730 from the
datasheet page.


Pins: 17

Input Mode:

1 Input the correct password then press the # key causes the solenoid to activate for 2 seconds.

2 .Input the incorrect password then press the # key causes:

  • 1st time = beep
  • 2nd time = beep
  • 3rd time = beeps for 60secs.

To disable the alarm, enter the correct code and press #

To change password:

  • Turn on SW1 the Memory switch
  • Key in the new password
  • Press the * key to complete entry.

If you wanrt to incorporate a siren or other alarm device, you can connect an inverter and driver to a siren as shown below. Pin 15 is the output for an alarm that will sound for 1 minute as well as the buzzer.

Written by Phil Townshend 2006
Back to Circuits page...

www.edutek.ltd.uk - Working Electronics For Students & Teachers