Two Transistor Logic Probe

This Two Transistor Logic Probe is a very simple circuit that can be implemented using two NPN bipolar transistors (T1, T2) in a Darlington configuration.

Doorbell for Deaf People.

When a deaf person is alone in a house, it would be very difficult to realize that someone is knocking on the door without any visual warning signal.

The main component for the Doorbell for the deaf circuit is the CMOS 4001 integrated circuit, which has four two input NOR gates

12 volt Power Supply using 7805

This 12 volt power supply circuit is simple and useful. Sometimes we need to get 12 VDC, but we only have 5 or 9 volts regulators (7805, 7809).

This Electric Current Flow Indicator gives a visual indication that there is a current flow. Possible applications for this small circuit may be: a battery charger, a voltage power source, etc.

In order to achieve our objective we place 3 rectifier diodes in series, which in turn are in series with the load / battery.

DC motor speed controller circuit using 555

This simple DC motor speed controller circuit, works with 12 volts and controls a DC motor using the 555 integrated circuit.

Dual polarity variable voltage source

This voltage source allows for two independent voltage outputs. One positive and one negative with a maximum amplitude of +/- 15 volts, an a current up to 1.5 amps.

Variable Dummy Load for Power Supply Testing.

How to test a voltage source to know if it is good or not?. What is the percentage regulation?.

4-channel audio mixer using LM3900 IC

4 operational amplifiers, some resistors and capacitors to implement an audio mixer with a very simple design.

12 to 5 volt converter circuit (for cars)

This 12 to 5 volt converter circuit for cars is very useful. It can be used to connect 5-volt electronic devices in a car with a 12v battery. (Automotive lead-acid batteries may have a voltage of up to 13.5 volts).

Light Operated Relay Circuit using LDR.

The LDR varies its value (ohms) depending on the amount of light that illuminates it. The more light less resistance and less light more resistance, The level of light that makes the relay activated is adjusted to the desired value with a potentiometer.

12V Auto Turn-off Battery Charger

This 12V Auto Turn-off Battery Charger with current limiter, uses only 4 transistors, will stop charging the battery when it has 14 volts between its terminals and limits the charging current to a maximum of 2 amps.

Car Burglar Alarm using two 555 timers

This is a simple Car Burglar Alarm circuit that uses a dual contact relay and two 555 timers, each of them working as a monostable multivibrator.

The alarm will activate the car horn, approximately 5 seconds after either door is opened and has not been properly closed.