Home > Circuit Blox > Motion Trigger
UPDATED:.12:00 29 June 2014

A This circuit will activate a device when it detects movement from a heated object - usually a person or animal.

The PIR has a range of at least 5 metres.

There are several ways of connecting a driver and load. Two are shown below.
Another method is shown in the Speed Controller #2

Takes about 30 seconds after power-up to settle and begin detecting.

Set the jumper to Range 2 - (max 2.5 secs)
For longer than 2.5 secs, select Range 3.

For reliable operation, use 3 x100nF capacitors between the motor connections and the case. Motors produce very strong electrical interference.

CIRCUIT BLOX - Circuit 1

The PIR produces a low output pulse when it detects a heat source and the LED comes on. This can be anything from 1 to 10 seconds depending on the heat source movement.

The low pulse triggers the Monostable which produces a timed high output pulse set by VR1

This output is connected to a Darlington driver which drives the load. In this circuit the supply voltage can be from 6v to 15v and a maximum current of 3 Amps. .

CIRCUIT BLOX - Circuit 2

In this version the operation is the same except a Relay is used to provide switch contacts for the load. The Motor Supply can be from 3v to 24v and a maximum current of 5 Amps.

The relay contacts can also be used to trigger other deviecs such as a speech recorder "Play" button to announce a warning.

PIR detection can be part of a topic of Sensors and Detection.

Topics could include:

  • Monostables - generating timed events
  • Calculating time using T=1.1CR
  • Relays and protecting from EMF
  • How coils can generate high voltages.


This can be used in a variety of projects:
  • Automated displays
  • Automaton models
  • Entry/warning announcements
  • Pet/bird scarer - motorised scarecrow

Alternative loads: Bulbs, Sirens, Buzzers..

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