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Simple Logic Probe

This simple logic probe has both LEDs on with no signal at the input but due to the nor gates connected to the probe, indicates correctly when a high or low signal is present. It also works correctly for pulse trains. Normally both LEDs are forward biased and therefore on, powered by the 12V supply. When a logic "high" is present at the probe, IC1a's output goes low sending IC1b's output high. This turns off LED1 but forward-biases (and turns on) LED2. Conversely, a logic "low" at the probe will send IC1b low, turning LED1 on and LED2 off.... [read more]

Nocturnal Animals Whisker

his circuit proved very useful in keeping away from a terrace or a porch some bats and other nocturnal animals. You can use it for similar or different purposes. The lamp illuminates at a 4-5 seconds delay and stays off about one minute and 15 seconds.... [read more]

Multicolor HD LED

Most PC enclosures provide only a single LED to indicate hard disk access, with the LED being connected to the motherboard via a two-pin connector. However, this LED only works with IDE drives, and if a SCSI disk controller is fitted, its activity will not be visibly noticeable. This small circuit remedies that problem using a multicolour LED. The activity LED for the IDE interface is usually driven by a connected device via one or more open-collector stages.... [read more]

Remote Doorbell Warning Switch

This circuit should only be used with the solenoid type chime doorbells, the electronic type that play tunes will not work here.... [read more]

Transformerless 5 Volt Power Supply

An increasing number of appliances draw a very small current from the power supply. If you need to design a mains powered device, you could generally choose between a linear and a switch-mode power supply. However, what if the appliance’s total power consumption is very small? Transformer-based power supplies are bulky, while the switchers are generally made to provide greater current output, with a significant increase in complexity, problems involving PCB layout and, inherently, reduced reliability.... [read more]

120VAC Lamp Chaser

This circuit is basically the same as the 10 channel LED sequencer with the addition of solid state relays to control the AC lamps. The relay shown in the diagram is a Radio Shack 3 amp unit (part no. 275-310) that requires 1.2 volts DC to activate. No current spec was given but I assume it needs just a few milliamps to light the internal LED. A 360 ohm resistor is shown which would limit the current to 17 mA using a 9 volt supply.... [read more]