Pulse sequence shaper. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer
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If you need a reliable and cheap source of a pulse train with a frequency close to 50 Hz and a fairly high frequency stability (about 0,5%), you can implement it using a pulse shaper from the mains voltage. Although these sources are inferior to quartz sources, for many applications (for example, for a timer) they are preferable, since they are distinguished by the simplicity and availability of components.
A diagram of one of the variants of such a pulse shaper operating as part of a time interval programmer is shown in fig. 1. The output circuit of the shaper is galvanically isolated from the network by a self-made optocoupler, consisting of a neon lamp HL1 and a phototransistor VT1. Resistor R1 limits the current through the neon lamp. Diode VD1 provides ignition of the lamp for only one half-cycle. Without the diode, the output frequency will be 100 Hz. A current amplifier is made on the transistor VT2.
The shape of the output pulses is close to rectangular (Fig. 2). Their duty cycle depends on the type of neon lamp used, but, as a rule, it is more than 2,7.
The neon lamp HL1 was used from the SVP-4 television program selector; any one with an ignition voltage of 70 ... 80 V is suitable. Instead of FT-1K, you can use a phototransistor FT-2K.
The neon lamp should be turned on so that its plate electrode is connected to the anode of the VD1 diode. The neon lamp and the phototransistor are mutually placed so that its light-receiving window is opposite the most brightly luminous zone of the lamp. The resulting optocoupler must be closed with an opaque cap that protects the phototransistor from external light.
Any diode VD1 is suitable with a reverse voltage greater than the ignition voltage of a neon lamp. It is possible to replace the phototransistor FT-1K with a photodiode (FDK-1, FD-10K, etc.), but this will require one more stage of current amplification.
When using the shaper in conjunction with TTL counters, its output signal must first be passed through the Schmitt trigger of the K155TL1 microcircuit, for example.
Author: I. Gorodetsky, Moscow
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