Separate ON-OFF Touch Switch

Providing a touch switch can be a cool way to make your circuit looks sophisticated. Moreover, providing separate touch pad for “on” and “off” function improve the safety, since touching one function doesn’t trigger opposite action to what we expect even at wrong assumption about current state. Here is the schematic diagram of the circuit: How this circuit works is […]

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First-Response Monitor with SCR (Quiz Alarm)

A pair of cross-coupled SCRs can be used to build a first-response monitor circuit, as shown in the schematic diagram below.  First-response circuit is popular in quiz application where the first contestant who press the button will prevent the other activate the button/alarm. The purpose of this circuit in quiz application is to ease the identification of the first one […]

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Relay Circuit Detects Light Change

Configured with capacitive coupling (by inserting a small cap between photo-transistor and bipolar transistor), this relay circuit will respond only to rapid changes  in light while ignoring normal gradual changes in ambient light. Rapid changes can be produced by light interruption or turning on/off lamps, while gradual light changes can be produced by clouds or  sunrise.  Here is the schematic […]

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Solid State Relay (SSR) with Optocoupler and Triac

This solid state relay (SSR)  equivalent circuit is made of optocoupler and a triac. This SSR circuit is also designed with  LED protection from overvoltage and reverse polarity. The diode 1N4002 is the reverse polarity protector, as it will short the wrong polarity to null the voltage with the help of 150 Ohm series resistor.  The 2N4403 transistor will protect […]

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