In many electronic projects, at some point, it may involve switching external circuits which have a very different voltage rating. For instance, if you want to switch relatively low voltage circuit, it would be recommended to use transistors. However, if you want to switch a high voltage AC circuit using low voltage DC signal, you will need another ways. One way is using relay. However, a typical relay would require at least 5V to energize its coil. If your low voltage signal does not reach such rating, you will need to amplify it. You can do so using a NPN transistor.
Suggested components needed:
7805CT voltage regulator, 1N4002 general purpose diode, 2N3904 NPN and SSR3A5V solid state relay (does not have any coil).
Here’s how:
If your low voltage signal is more than 0.7V, you should add a resistor in series so that it stays below the maximum rating. Then, connect this 0.7V signal to the base of the transistor. At the collector, insert your 5V or your desired amplified signal. After which, you can connect the relay coil positive to the emitter of the transistor and the relay coil negative to the ground. As a standard procedure, a standard diode should be connected parallel to the relay coil to prevent damage by the back current.
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