Home > Circuit Blox > Motion Trigger

UPDATED:.21:23 02 January 2019

This motion trigger has been used often to animate models and signs. The circuit is just 3 blocks and will activate a device when it detects movement from a heated object - usually a person or animal.

Connect the relay contacts in line with a motor and you get an animated model.

The PIR has a range of at least 5 metres.




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

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, to help suppress the high voltage spikes produced by emf.

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. However if the PIR signal is active for longer than the timed period, then the monostable timing is ignored. If the timing is critical, then use the Circuit 2 option below.

This output from the monostable is connected to a relay with a transistor driver

The switch contacts of the relay are used in series with the motor supply, to activate the motor. The Motor Supply can be from 3v to 24v and a maximum current of 5 Amps.

Motor suppression

Another animated figure

In the dinosaur model, there is a cam connected to the motor to move the tongue. This cam also pushes against a microswitch which is wired to the same connections as the relay. When the relay deactivates, the microswitch contacts take over until there is a dip in the cam, thus creating a precise stopping point.


Below is an alternative circuit that has the addition of a spike generator after the PIR sensor. This prevents the PIR sensor overiding the time set by the Monostable so the motor is always activated for a precise amount of time, regardless of the signal from the PIR.

Set the spike generator polarity to "-".

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

This Shrek(TM) head made from vacuum forming, has Circuit Blox inside. When anyone appraoches the face lights up and a pre-recorded message plays.

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..