Random circuits

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Circuit Board Checker

This little circuit indicates the basic integrity of a printed board, detecting 0V, positive supply voltage from less than 3V to 30V and floating parts. If the probe is floating, as it would be in a broken track, then both LEDs barely light up, since there is no current to drive the transistors, but if the probe touches 0V or a positive voltage one or other lights. A digital signal should light them in proportion to the mark-space ratio whereas the output of a circuit oscillating at a frequency rate below about 20Hz will cause the LEDs to flicker alternatively. The LEDs will illuminate always at a constant intensity, no matter the voltage supply used, because they are fed by a very simple FET constant-current generator (Q1).... [read more]

A 4-Digit Alarm Control Keypad

This Keypad is suitable for the Modular Burglar Alarm. However, it has other applications. Pressing a single key will energize the relay. Entering the Four-Digit code of your choice will de-energize it.... [read more]

Discrete Virtual Ground Circuit

Here is the simple virtual ground circuit based on discrete components. This simple design comes from miniaturization guru Sijosae. Is to make a buffer from generic discrete components. The transistors can be most any complementary pair of small-signal transistors. Suitable alternatives are the PN2222A and PN2907A. The diodes are generic small-signal types. An acceptable alternative is the 1N914. This circuit has better performance than a simple resistive divider virtual ground, and the parts cost is lower than for any other circuit mentioned here. It is, however, the least accurate of the buffered virtual ground circuits.... [read more]

Simple Lie Detector

Here's a simple lie detector that can be built in a few minutes, but can be incredibly useful when you want to know if someone is really telling you the truth. It is not as sophisticated as the ones the professionals use, but it works. It works by measuring skin resistance, which goes down when you lie.... [read more]

Picoammeter circuit with 4 ranges

This circuit uses a CA3420 BiMOS op amp to form a picoammeter with 4 ranges. The exceptionally low input current (typically 0.2pA) makes the CA3420 highly suited for use in a picoammeter circuit. Input transient protection is provided by the 1 megohm resistor in series with the input.... [read more]

Protection For Voltage Regulators

People often forget that many voltage regulator ICs have an upper limit (usually 35 V) on the input voltage they can handle. That applies primarily to types with a fixed output voltage. Adjustable voltage regulators also have a maximum voltage specification, in that case between the input and output (commonly 40 V). The input voltage must thus be limited to that level in a fault situation in which the output is shorted.... [read more]