This low pass filter has attenuation of 5.5 dB and the frequencies depend on the values of the components in the case 1 kHz.
This circuit is from 1964 documentation but can be easily implemented based on current components....
This Power supply sources from 500 to 1,000 mA according the transformar and was found in a...
This photo-relay uses a Darlington stage to increase sensivity. Any photo-transistor can be used....