This DC power supply controller is controlled by pulse width modulation PWM, generated by the circuit around timer IC2 7555 according to the formula:
Duty cycle = ((R1 + (VR1/2)/(R1+R2+VR1)) x 100%
Diode D1 is used to bypass resistor R2 and half the effective resistance of the “lower” section of VR1, allowing the duty cycle to be varied from 0.01% to 99.9%.
Read the rest of this DC power controller circuit.

There are two main types of battery charger – constant voltage and constant current. Both have their advantages and disavantages. For constant voltage, the battery cannot be overcharged but the charging rate is slow. Constant current mode can charge batteries more swiftly but there is the danger of overcharging them.
The dual mode battery charger circuit featured here was designed to combine both modes, but without their disadvantages, for use with a 6V sealed lead-acid battery.

Read the rest and view the dual mode battery charger circuit

The metal detector circuit shown here must represent the limits of simplicity for a metal detector, yet the design works surprisingly well. It uses just one 40106 hex Schmitt inverter IC, a capacitor and a search coil – and of course the batteries. A lead from IC1b pin 4 needs to be attached to a medium wave radio aerial, or it should be wrapped around the radio. It can be used even like those hand held metal detectors.

This house FM transmitter for your stereo or any other amplifier provides a good signal strength up to a distance of 500 meters with a power output of about 200 mW. It works off a 9V battery.

The audio-frequency modulation stage is built around transistor BF494 (T1), which is wired as a VHF oscillator and modulates the audio signal present at the base. Using preset VR1, you can adjust the audio signal level.