Street Light Control CircuitNon-Inverting Schmitt Trigger
A light-sensing street-lamp switch made simple: two NOR gates, a handful of resistors and some hysteresis.
- Reading
- 02 min
- Figures
- 06
A simplified version of the smart switch inside a street lamp: a non-inverting Schmitt trigger, built from resistors, an LED and NOR gates, that turns the “street light” on and off with the light around it. Hysteresis gives the switch a luminosity tolerance, so it flips decisively and filters out noise.
- Disciplines
- Electronics
- Circuit Design
- Physics
- Experimentation
- Tools
- Breadboard / MC14001BCP NOR-gate IC / Light-dependent resistor / Bench power supply / Multimeter / Light meter
Have you ever noticed how, on grey, gloomy days, street lamps turn on much earlier than on sunny ones? Street lights have a built-in smart switch, though it tends to be a very complicated circuit board.
Out of my endless curiosity for engineering, I set out to simplify it. With some resistors, an LED and NOR gates, I built a non-inverting Schmitt trigger with hysteresis: a circuit that turns the street light — here, the LED — on and off automatically according to the luminosity around it.
A decisive switch
A Schmitt trigger has two thresholds instead of one. As the input voltage rises past the upper threshold, 3 V, the output snaps high to 5 V; it only drops back to 0 V once the input falls below the lower threshold, 2 V.
The band between them — the dead band — is the circuit’s luminosity tolerance. Small changes in light inside it are ignored, which gives a more decisive switch and filters out noise.
2 V
lower switching threshold (VLT)
3 V
upper switching threshold (VUT)
5 V
supply voltage, and the output high (VT = VH)
Under controlled light
The circuit was built on a breadboard around an MC14001BCP NOR-gate chip, with the LED standing in for the street lamp and a light-dependent resistor as its eye.
To test it, the LDR went inside a photo light box, laid on foam so its exposed surface was flat, with a light meter’s sensor placed as close to it as possible. The light meter measured the luminosity inside the box, a digital multimeter measured the LDR’s resistance, and a voltmeter confirmed that the supply held at 5 volts.
Fig. 03The “street light” on.
Fig. 04Inside the light box: the LDR on foam, the light-meter sensor beside it.
Fig. 05A light meter reads the luminosity; a multimeter reads the LDR’s resistance.
Fig. 06The bench: power supply, a voltmeter holding VT at 5 V, and the control circuit.
