Red LEDs function as light sensors

Geoff Nicholls

You can build simple LED illuminators using the red LED to sense ambient-light level.

Ordinary red LEDs normally function as light emitters, but they can also function as photosensors. A single LED can even function as both a light emitter and a light detector in the same circuit. The basic idea is to pulse the LED, using the on-time to light it and the off-time to sense the photovoltaic current from the ambient light that the LED sees.

This circuit functions as a night-light. The LED stays off during daylight and turns on when the ambient light level drops

Figure 1’s circuit functions as a night-light. The LED stays off during daylight and turns on when the ambient-light level drops. The 7555 CMOS timer is a monostable one-shot, which triggers when Pin 2’s voltage is less than one-third of the supply voltage. R1 and R2 form a voltage divider, which keeps the cathode of the LED just below the trigger voltage. When the ambient-light level is sufficient, the LED develops several hundred millivolts, which add to the R1/R2-junction voltage and keep Pin 2 above the one-third-trigger level. In this state, the Pin 3 output of the 7555 approaches 0V, and the 1N914 diode becomes reverse-biased, allowing the LED’s photovoltaic current to flow into Pin 2’s trigger input.

When the ambient-light level drops low enough, the LED voltage falls, and Pin 2 goes below the trigger level. The 7555 then generates a one-shot pulse, the 1N914 becomes forward-biased, and the LED lights up. At the end of the timing period, which R3 and C1 set, the monostable resets and discharges C1. The monostable is then ready for another cycle. The LED then briefly turns off during this interval, which allows it to again sense the ambient light.

The LED in this circuit flashes slowly when the ambient-light level is high and turns off when the light level drops

The circuit in Figure 2 functions as a day-light; the LED flashes in bright light and stays off in low ambient light. The 7555 provides astable operation and slowly flashes the LED through the 1N914 diode as long as Pin 4’s reset input is greater than approximately 600 mV. If the ambient light is too low, the LED cannot generate enough voltage at Pin 4, and the 7555 output remains near 0V, preventing the LED from turning on. The LED operates as a light emitter when Pin 3’s output is high and as a sensor when Pin 3’s output is low.

These circuits require no current-limiting resistor. The timer IC must be a CMOS type because, to operate correctly, the circuit design requires low input currents. The prototypes use Intersil’s ICM7555 devices.