This circuit was taken from an article in Electronics Now magazine in February 1995. The power supply must be dual and other operational amplifier can be used.
We find this circuit in an old documentation, but it can be implemented with modern equivalent...
This circuit eliminates transients that are 10% above the zener voltage. The transistor must be in...
A solution for using more than 6 channels in a tone-modulated system that uses LM567 as decoders, is to...