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UPDATED:.12:09 29 June 2014

This circuit uses a PIR to detect people entering an area and switches on a light but only when it is dark.

It is important that the light does not shine on the LDR as this will cause the circuit to switch on and off quickly - see below for explanation.

Circuit Voltage: 6 to 9 volts

Dark Sensor:
This is the Light Sensor connected with the supply reversed.

Set Range 3, (2.4s - 110s), use VR1 to set time light is on for.

Comparator Switch:
Set Schmitt option to ON, use VR1 to set the level of darkness to enable the lgiht to come on.



The PIR sensor output goes low when a presence is detected.

The low signal is connected to the -Trigger of the Monostable and will commence timing if the -Reset input is high. The duration of this can be altered using VR1 and can time up to 110 seconds.

The output from the Monostable is connected to the Relay which activates when the Monstable output is high. The relay contacts can be used to switch a light or lamp.

The Light Sensor is connected upside down to create a Dark Sensor, whose output rises the darker it gets.

The output from the Dark Sensor is connected to a Comparator Switch which will send its output high when the voltage from the sensor reaches Vref, set by VR1. When the output goes high, (and it has reached a certain level of darkness), this enables the Monostable to function. When the light level increases, the voltage from the sensor reduces, the Comparator switches off sending the "-Reset" input low, thus disabling the Monostable.


Operational Amplifiers used as a Comparator.

  • How to crerate a voltage reference with a potential divider
  • Using a potentiometer as an adjustable potential divider

Monostables producing timed outputs

  • Calculating time using T=1.1CR

Other output devices could be connected - such as a siren so that an alarm is sounded only in the hours of darkness.

This could also be used indoors so that a room is secure when the light is switched off. A thief however would not know this and is unlikely to turn on a light when a siren is going.

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