Random circuits

Here are some of over 800 projects from our free circuit diagrams database. For more, try browsing categories menu on the left.

Simple Short-Circuit Detection

This circuit is suitable in every situation where over-current protection is required. Here we give an example from the model train world. Every seasoned model train enthusiast knows that there is nothing worse than having to find the cause of a short-circuit. On a small model railway with one locomotive it is obviously fairly easy, but on large layouts all locomotives stand still when there is a short and then you have to check each one in turn to find the culprit.... [read more]

Fire Alarm Using Thermistor

In this fire alarm circuit, a Thermistor works as the heat sensor. When temperature increases, its resistance decreases, and vice versa. At normal temperature, the resistance of the Thermistor (TH1) is approximately 10 kilo-ohms, which reduces to a few ohms as the temperature increases beyond 100 C. The circuit uses readily available components and can be easily constructed on any general-purpose PCB.... [read more]

Capacitive Sensor Circuit Diagram

The purpose of this circuit is to animate shop-windows by means of a capacitive sensor placed behind a post-card-like banner. The card is placed against the glass inside the shop-window, and the visitor can activate the relay placing his hand on the card, from the outside. Especially suited for toy-shops, the circuit can activate model trains, small electric racing cars, lights etc. Further applications are left at user's imagination. Adopt it to increase the impact of your shop-window on next Christmas season!... [read more]

Digital Voltmeter

The ICL7107 is a 3 1/2 digit LED A/D convertor. It contains an internal voltage reference, high isolation analog switches, sequential control logic, and the display drivers.... [read more]

Switchable Output Crystal Oscillator

This oscillator circuit permits crystals to be electronically switched by logic commands. The circuit is best understood by initially ignoring all crystals. Furthermore, assume that all diodes are shorts and their associated 1kO resistors open. The two 1kO resistors at the non-inverting input of IC1 (LT1016) set the output to half the supply, ie, +2.5V. The RC network from the output to pin 3 sets up phase-shifted feedback and the circuit looks like a wide-band unity gain follower at DC. When crystal X1 is inserted (remember, D1 is temporarily shorted) positive feedback occurs and oscillation commences at the crystal’s resonant frequency.... [read more]

Repeating Interval Timer

This circuit has an adjustable output timer that will re-trigger at regular intervals. The output period can be anything from a fraction of a second to half-an-hour or more - and it can be made to recur at regular intervals of anything from seconds to days and beyond.... [read more]