Home > GCSE Projects > Pig Sentry

Modified: 21:07, 26 October 2013

Circuit and board designed by Phil Townshend 1994

This project was designed for a student who wanted to know how many times their family went in their room. The sensor, a PIR module detects someone walking by which increases a counter. This can be displayed by pressing a switch, mainly to preserve battery life. If possible use CMOS versions of the 555 timer to increase battery life.

The circuit is simple and based upon a 555 monostable and a counter/7 segment decoder I.C. 4026.

Below is the PCB layout (better version with download) showing the connections to the board. At the top is the facility to connect a PIR and is pin compatible with the Maplin PIR module (YD85G). However be sure to leave the +Trig input disconnected if using the PIR module to prevent possible damage to it.

If you are using another type of trigger (PTM switch, pressure pad, relay etc) there are 2 options. A positive trigger (+Trig) and a negative trigger (-Trig). You can use either of these to increment the counter by connecting the switch between +v and +Trig or 0v and -Trig. The monostable is set to about 1 second and prevents multiple counting from contact bounce. This means it cannot count faster than one pulse per second. To reduce increase the rate it can count reduce R2 and C1 using t = 1.1 x C x R

If not using a switch to activate the display and want it on all the time, link the two connection labelled "DISPLAY ON" witha piece of wire. This will drain a PP3 battery within hours so you might need to use a power supply.

The polarity of the supply is protected using 1N4001 diode. An LED can be connected to the "LED" output if required.

Picture of Maplin PIR sensor YD85G. Wires can connect directly to the PCB. Ensure they are the correct way round!
The 7 segment display used is a good quality high brightness from Kingbright used in a lot of projects. Rapid order 57-0129. Also avaiable in green.

A complete kit or just PCB are available from our shop. - (not including sensor)

Above: Vacuum formed case with button pressed to enable the display. In this case LED's were added to enhance the design.
Shown right the 7-segment display is connected via ribbon cable rather than directly mounted to the PCB. The other ends are soldered directly to the 7-segment display.
(Note: Different PCB shown)
To extend the display away from the PCB, solder in turned pin SIL sockets where the 7 segment display normally fits. Then plug the 7 segment display into the turned pin sockets. This can make mounting the PCB and having the display showing through the case a bit tricky but means no connecting wires.
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