Electronic – Flashing a light when a circuit breaks

Basically the way these circuits work is by relying on the fact that soil conducts electricity, with a resistance of a few hundred kOhm. When it gets wet, the resistance changes allowing the moisture level to be detected by measuring the resistance of the soil.

The measurement is usually achieved using an amplifier circuit which produces a voltage depending on the resistance of the soil. This can be done using an Op-Amp circuit for example, or even just biasing a BJT with the soil probe such as in the example circuit below (Source of image and circuit description here) where S1/S2 are probes in the soil, and the voltage at S1 varies with moisture level.

The LED is almost certainly not in line with the sensor, but is rather turned on or off by the measured voltage. So basically if you want an LED to have the opposite indicator, you would just need a simple inverter circuit driving the second LED.

It is best to feed the string which in your case has normaly 3 leds in series with a current source this stops led current rising steeply with increasing supply volts and stops current falling significantly unless supply volts go below about 10 which is unlikely NOW you can safely place a zener across each LED which is just above the max LED forward volts Example 3V9 for white led This is all cheap because the current source could be a two terminal IC or discrete transistor circuit and the zener doesn't have to be high power In fact all parts can be SMD so parts count isn't a penalty