Opto-Coupled Discrete Pulse Stretcher

This is a pulse stretcher circuit using optocoupler. This circuit uses a 4N26 opto-coupler combined with a standard one-shot circuitry. This circuit is triggered by pulse about 3μs (minimum width)  at 15mA. The output’s amplitude of this circuit is not depend on the input’s amplitude. The output pulse amplitude is a function of (or proportional to) supply voltage on the […]

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Bandpass Filter for Bell103A Originator Modem

Bell103A modem standard specify that the originator (the caller) uses 1070Hz and 1270Hz frequency shift keying for transmission, and listen for 2025/2225Hz FSK transmitted by the answering modem. This bandpass circuit filter out any frequency beyond 2025-2250 Hz to improve originator’s decoding of the received FSK signal. Here is the schematic diagram of the circuit:

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Five-Decade Dynamic Light-Level Meter

This is a Portable light-level meter circuit with five-decade dynamic range. This uses  a single cell battery as the power supply. To calibrate this circuit, use an appropriate illumination, then adjust R2 so that the output equals the reference. To make this circuit give output meter follows standard light meter, adjust R2 and R6. Maybe there is an error at […]

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Single Transistor Variable 0-180 Deg Phase Shifter

The schematic diagram below shows a variable 0-180 degree phase shifter circuit. This circuit can provide  phase shifted output (compared with the input) between 0 and 180 degrees for frequencies up to 3MHz with same amplitude (unity gain or gain=1.) This circuit uses inexpensive NPN transistor, 2N3904, but almost any small signal transistor type can be used. Using R5 and […]

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