A common circuit such as a night light may use a CdS photocell to turn it off during daylight hours and on during night time. The photocell is connected to a circuit that senses the voltage and turns the rest of the circuit on and off at a threshold point. The photocell is in series with a trimmer resistor that allows adjusting the sensitivity. During daytime the light causes the photocell to drop below 1 thousand ohms, for example, and at night the photocell resistance may increase to many times that much, 50 thousand ohms, for example. The night time still has some ambient light, even though it is not daytime. So the trimmer resistor might be adjusted to 10 thousand ohms, and the circuit works just fine.
The circuit has a supply of 5 volts, for example, and during the daytime the total resistance of the CdS photocell and trimmer resistor might be less than 11 thousand ohms. Five volts divided by 10k equals 1/2 milliamp. This current is being drawn from the power supply for about half the 24 hour day on average, and is not a problem with a power supply that is plugged into the wall.
But let’s say the power supply is a few AA cells. The total current from the batteries during night time when the circuit is running might be only 5 milliamps, so during the day when the circuit is off, the 1/2 milliamp is about ten percent of the total, and this will reduce the battery lifetime somewhat.
It would help if we could reduce the current through the photocell and trimmer resistor during the night time. But how? If we increase the trimmer resistor, then the ambient light at night may cause the circuit to falsely go off, and we don’t want that to happen.
One way is to reduce the amount of light that hits the photocell at any time. This can be done by putting something that blocks most of the light in front of the photocell. However, one thing is important: the photocell needs more than that.
Years ago most of the CdS photocells were sealed in a metal can, which blocked all of the light except what came in through the front window. Nowadays, most of the inexpensive photocells are made on a white ceramic disk, which is does not block the light coming from the back – a small amount of light still gets through. So if something like a piece of sunglass lens blocks the light in the front, the rest of the photocell has to be enclosed in something that also blocks the light from the sides and back.
Another way to reduce the current through the photocell is to increase the trimmer resistor resistance. This cuts down the current, but it also reduces the amount of voltage change between daylight and night time. This may make the circuit more complicated and cause the circuit to be more sensitive to noise.
One good design addition is an electrolytic capacitor across the photocell. This could be in the range of 10 to a few hundred microfarads. The capacitor slows down the change in voltage, so that if a shadow is cast on the photocell for a short time, it won’t trigger the circuit to come on. One characteristic of larger electrolytics is they may have some leakage, so if the photocell is very high resistance, the capacitor’s leakage may cause the voltage across them to drop. This is something that should be taken into account in a low current, high resistance photocell design.