This Linear Technology circuit can measure currents from 100 pA to 100 uA. The circuit is from the 80's, using a component of the era. Transistors are general purpose types.
An stable multivibrator with 50% of duty cucle can be mounted using the 555 as shown by the...
This circuit uses a Norton operational amplifier to generate 1 kHz pulses with a 1 to 60...
This circuit, with a configuration like to many that we can find in this section, was suggested by a...