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The circuit below responds to sound pressure levels from about 60 to 70 dB. The sound is picked up by an 8 ohm speaker, amplified by a transistor stage and one LM324 op-amp section. You can also use a dynamic microphone but I found the speaker was more sensitive. The remaining 3 sections of the LM324 quad op-amp are used as voltage comparators and drive 3 indicator LEDs or incandescents which are spaced about 3dB apart. An additional transistor is needed for incandescent lights as shown with the lower lamp.... [read more]

Here is a simple thermostat circuit that can be used to control a relay and supply power to a small space heater through the relay contacts. The relay contacts should be rated above the current requirements for the heater.... [read more]

With this circuit we can amplification selectively the high signal of picture frequencies [Video] with result bigger clarity than this. The circuit should be placed between a Video unit and the reception Scart or the plug Video input of television receiver.... [read more]

It's a classic designing of final amplifier 35 W, with two EL34 in push-pull, from the Siemens and Halske, with year of designing 24/3/1953 and code SV410/1. The amplifier it worked from 1954 until 1989, whenever it came also out except operation, with mean of operation 15 hours per day.... [read more]

This is a great intercom circuit that can be used in many ways. It uses 22V to operate and maybe it will work at a lower voltage (you can try it). For input/output it uses a loundspeaker (20-45 Ohm) on each side.... [read more]

Here they exist two regulator circuits, that use the IC L200, as regulator of voltage and current, the company SGS-Thomson, which give these circuits. In circuit Fig.1, we can regulate the output voltage, with the RV1, while in the Fig.2, we can regulate also output voltage – current, with the TR2 and TR1, respectively. More information on characteristics for L200, you can see in list datasheets. Shortly will be added also certain other useful circuits with L200.... [read more]