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This circuit is very similar to Repeating Interval Timer No.2 . However - the addition of a light dependent resistor means that the operation of this timer can be limited to the daylight hours. A variable resistor (preset) lets you choose the level of darkness at which the timer will cease to function.... [read more]

The heater element (not shown) is connected in series with two back to back 16 amp SCRs (not shown) which are controlled with a small pulse transformer. The pulse transformer has 3 identical windings, two of which are used to supply trigger pulses to the SCRs, and the third winding is connected to a PNP transistor pair that alternately supply pulses to the transformer at the beginning of each AC half cycle. The trigger pulses are applied to both SCRs near the beginning of each AC half cycle but only one conducts depending on the AC polarity.... [read more]

The thermistor used has a resistance of 15k at 25 degrees and 45k at 0 degrees celsius.... [read more]

Short circuits or broken pcb tracks can be easily recognized by means of a Multimeter, but this tool can give wrong results when testing the efficiency of a transistor or diode, unless the device under test is unsoldered and removed from the pcb. A further shortcoming affecting such way of testing is the necessity to keep firmly the probes on the pins of the device under test and at the same time to turn the head continually to read the Multimeter display.... [read more]

This low-cost, general-purpose electronic hearing aid works off 3V DC (2x1.5V battery). The circuit can be easily assembled on a veroboard. For easy assembling and maintenance, use an 8-pin DIP IC socket for TDA2822M.... [read more]

This circuit was designed to detect the approximate percentage of salt contained in a liquid. After careful setting it can be useful to persons needing a quick, rough indication of the salt content in liquid foods for diet purposes etc.... [read more]