Introduction: Transistor LED Dimmer

About: Recycled Components is a small group of people in Calamba, Misamis Occidental that designs circuits from recycled components. All circuits are made and tested in Australia.

This Instructable shows you how to make a simple transistor LED dimmer.

There is a cheaper alternative:

However, the circuit in the link above can only drive low-current and low-power LED lights. This circuit can drive higher currents and higher-power LEDs. However, even this circuit has a limit. Other LED dimmers are shown here:


Components: Bright LED - 2 (you only need one but you might burn one and the circuit can drive more than one LED), resistors that are shown in the circuit, piece of cardboard or matrix board, 1 kohm or 10 kohm potentiometer, power source (AA or AAA batteries and battery harness), PNP or NPN BJT transistor, heat sink, heat transfer paste, insulated wires or 1 mm metal wire.

Tools: wires tripper, pliers, scissors.

Optional components: solder, box ( or cardboard).

Optional tools: soldering iron, multimeter.

Step 1: Design the Circuit

The circuit shown is for PNP transistor. You can use an NPN transistor if you turn the circuit upside down. The emitter terminal is showing the direction of the base and emitter current.

Electrons always flow from the negative battery terminal to the positive battery terminal. However, we can assume a conventional analysis current to simply our design. We can assume that there is such a thing as a conventional current that flows from the positive battery terminal to the negative battery terminal.

In a PNP transistor:

- the current flows from the emitter to the collector.

- the base current is flowing out of the transistor.

In an NPN transistor:

- the current is flowing from the collector to the emitter,

- the base current is flowing inside the transistor.

The collector current (transistor output current) needs to be 10 mA because this is the minimum current that our LED needs (some bigger and brighter LEDs need 15 mA or even 20 mA). Assuming that the minimum current gain is 20 the input current entering the transistor base is 500 uA or 0.0005 A.

Thus we can calculate the Rb resistor.

RbMax = (Vs - Vbe) / IbMin = 2.3 V / 0.0005 A = 4600 ohms or 4.7 kohms.

Suppose we want to add an additional LED with a 100-ohm resistor in parallel or two LEDs in parallel with a 50 ohm Rd resistor (keep in mind that if two LEDs are placed in parallel one might be slightly darker than another). Then the output current will be 20 mA. Thus we need to calculate the maximum Rb value for different output currents or different amounts of LEDs connected to the transistor.

Ic = 20 mA: RbMax = 2.3 V / 0.001 A = 2300 ohms or 2.2 kohms

Ic = 40 mA: RbMax = 2.3 V / 0.002 A = 1150 ohms or 1 kohms

The maximum output current is 40 mA for general-purpose transistors. Thus we should not reduce the Rb value below 1 kohms, unless we use a power transistor. Then we might need to reduce the Rv to at least 500 ohms. This would lead to power loss.

The maximum proper potentiometer value for one 10 mA LED output is 20 kohms. For higher potentiometer values, the LED will not turn ON until you reach about 99 % of the full-cycle setting. This would not be a linear control system. I also why I did not use a MOSFET or JFET transistor for this circuit. Those transistors are not linear, especially the MOSFETs.

Step 2: Simulations

I have drawn the circuit in the old PSpice software version 9.1 student edition to reduce drawing time. A bright LED is modeled with three general-purpose diodes because this software did not have a LED component.

Simulations show that the circuit will work. The LED is receiving almost 10 mA that it needs.

Step 3: Build the Circuit

I did not use a soldering iron. I twisted the wires together. You can build this circuit on a piece of cardboard or plastic. Cardboard is better than plastic. You can even build a bird nest circuit. However, this circuit is not very reliable unless you secure it to a hard material such as wood or packaging foam. However, the best option is a matrix board in my opinion and the worse option is making a PCB (printed circuit board).

Step 4: Put the Circuit in a Box

I used a cardboard box. You can use a plastic box or lunch box. However, a cardboard box is the best choice and you do not even need a box.

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