The Molecular Universe

A while back I was inspired by circuits like this: https://bestengineeringprojects.com/all-in-one-tester-circuit/ to make a transistor based continuity tester that uses red and green LEDs to indicate test results. It turned out that I did not succeed in getting the published circuit to work satisfactorily. But with some modifications, I made a closely related circuit that works. The circuit is shown below.

All in One Tester Circuit (click for a larger image - and click again to return to the current size)

The major changes were to avoid using a two color LED, and instead using two LEDs. I also found it necessary to change the circuit around a bit. I am not sure why this should be the case. I wanted the circuit to work at 4.5 volts and perhaps this was the difference.

Here is how the circuit works. When the circuit is powered on, the second NPN transistor is biased into conduction by the 180k resistor, and the red LED is illuminated.

When the leads are connected together, the first NPN transistor turns on, this turns of the second transistor. The green LED is then illuminated, and the red LED is turned off because the second transistor is turned off.

The circuit can be used as a continuity tester, it can test electrolytic capacitors (see below), and it supplies enough current to light LEDs, which can be useful for checking their operation.

Checking an electrolytic capacitor proceeds as follows. The positive and negative leads of the tester are applied to the positive and negative leads of the capacitor. The current flow though charging the capacitor will initially turn on the first transistor and therefore the red LED. However, when the capacitor is charged, current will cease flowing, the first transistor will turn off along with the green LED, and the second transistor and red LED will turn on. This color change over confirms that the capacitor is working correctly.

The circuit applies around 1 milliamp at about 3.5 volts (approximately) to whatever it is testing. So, it can make LEDs glow dimly (and safely), if you want to test such devices.

So far it has proven quite useful. And it fits into a Walmart sized TicTac mint box, which is a definite plus!

All in One Tester in Operation (click for a larger image - and click again to return to the current size)