Introduction: Recycled Transistor Amplifier
This circuit is used to amplify a signal from a sensor. This could be a photodiode, optocoupler or LDR (light-dependent resistor). You can use it as a water/liquid sensor, touch activation.
This circuit has a linear DC amplification. Thus when connected to an LDR this circuit can act as a control system to compensate for dim light. However, the circuit lacks power system efficiency because the transistors are on all the time.
I used 2N2222, NPN BJT transistors that can be found in many old electronic circuits. However, you might need to use a power transistor with a heat sink.
If you have PNP BJT transistors you can make the following circuit: https://www.instructables.com/id/Transistor-Sensor...
BJT NPN transistors - 7, LEDs/bright LEDs - 3, 1 kohm resistor - 3, diode (general purpose), 1 Megohm variable resistor, high power 100 ohm resistor - 2, 100 ohm resistor - 1, 10 kohm resistor, 47 uF capacitor, AA/AAA 3 V battery holder, 2 AA/AAA batteries or power supply.
Optional: power transistor, speaker, relay, crocodile clips, soldering iron, solder, multi-meter, photodiode, LDR (light-dependent resistor).
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Step 1: Create the Circuit
The circuits around the transistors represent the heat sinks. If you are connecting a relay or a speaker then you need a high power transistor. There is also an option of connecting more transistors in parallel.
Although I did state that the circuit is linear we must not forget that when the LED is on it creates a nonlinear loading effect on the transistor driver. You will need to disconnect the LED driving transistor (Q4a) from the two remaining transistors (Q4b and Q4c).
LED current = (3 V - 2 V)/100 ohms = 10 mA. For two LEDs you use a 47-ohm resistor. Do not increase the power supply. 10 mA current is maximum. You can try using high resistance values. (5 V - 2 V) / 10 mA = 300 ohms or 330 ohms for LEDs or 180 ohm for two LEDs.
Step 2: Build the Circuit
I made a circuit without Rd3a resistor. However, I connected Rd3b to Rd1. I found Rd3a does not really affect the output waveform from another circuit that I made.
The circuit works as a receiver to the remote control.