• Transistor Light Dimmer

    06/23/2020 at 11:34 0 comments

    I created a circuit for this article based on the following links:

    Use one transistor to drive an LED: 


    Used three transistors to drive a 0.3 A light bulb:


    My circuit uses two transistors to drive an old 0.1 A (100 mA) 12 V motorbike signalling light bulb with a 9 V battery.

    You can see my circuit working in this video:

    However, there are also oscillation/PWM light dimmers that are more efficient:



    Step 1: Design the Circuit

    I drawn the circuit in PSpice simulation software that also allows simulation:

    My circuit also allows linear input control that the other circuits (shown in the links) do not allow.

    The Q2 PNP transistor acts like a mixer circuit mixing two inputs:

    1. Variable resistor control.

    2. Input channel (Q1 transistors) control.

    Two Q1 transistors are used reduce the maximum power input current (base current) for each transistor that can occur due to high inputs voltages.

    The Cs and Rs is a power supply RC low pass filter used to filter power supply oscillations.

    Calculate the frequency of the low pass power supply filter:

    fl1 = 1 / (2*pi*Rs1*Cs1) = 1 / (2*pi*100 ohms*(470*10^-6 F))

    3.38627538493 Hz

    For more information on design you can read the links above.

    Step 2: Make the Circuit

    I only used a soldering iron for the power transistor. I used an old BD140 PNP power transistor from an old VCR (Video Cassette Recorder). It must have been used to drive the tape motors.

    I did not implement the power supply filter. This circuit is optional.

    Step 3: Encasement

    I used a nail to create the hole and widened with scissors:

    Step 4: Power Source

    9 V battery proved to be sufficient for the 12 V light motorbike signalling bulb.

    Step 5: Testing

    My circuit has good control over the bulb brightness because I used only two transistors instead of three.

  • Drawing Robot

    06/09/2020 at 05:32 0 comments

    This website will show you how to make a drawing robot.

    You can see my design implementation working in this video:

    I thought of this idea after reading this article:


    I attached a 1.5 V battery to my old robot that made many years ago shown here:


    Step 1: Attach Charcoal

    I tried using pencil or pen but my robot could not apply enough pressure even with blue tack used to increase the weight of the robot. Insufficient pressure/force applied to pen/pencil would cause my robot to move/spin without leaving any circles/lines/marks on the piece of paper.

    You can try using a brush with ink/paint.

    Step 2: Testing with Charcoal

    My Pentax camera is cheap and old, thus cannot capture fast movements.

    Slow Rotation:

    Medium Rotation:

    Fast Rotation:


    Step 3: Testing with Chalk

    I tried my robot outside:

    In real like my robot is not as fast and not as powerful as your see in my videos.

    You can increase the speed of this car/robot (if you are making a similar toy) by increasing the number of AA/AAA batteries connected in series and reducing the size of the batteries. If you are using a resilient motor (one that can handle higher voltages), then you can try using a 9 V battery.

  • Turn On In The Dark Light

    03/29/2020 at 10:28 0 comments

    This article shows how you can make a simple bright LED turn ON in the dark light with just three transistors.

    This circuit was made many years ago. About ten of those circuits have been made and send to the Philippines for testing:

    There is transistor and a 100 ohm resistor behind the yellow LED that you do not see in the photo. I use a typical LED because I did not have a bright LED in my inventory.

    You can see the circuit working in this video:

    You can build this circuit on a piece of cardboard or plastic.

    This circuit only turns ON the LED in complete darkness. Other more complicated circuits allow more precise control to compensate for dimness of light:



    However, those circuits cost more money and are harder to implement. Such precise control is not needed for simple turning ON in the dark light device.

    I never tried implementing this circuit with a MOSFET. This could be challenge for you.

    Designing the Circuit

    I have drawn the circuit via online https://easyeda.com software. The photo diode component was missing from the component list. Thus I used a general purpose diode.

    Rc1 and Rc2 resistors ensure that this circuit is OFF when the transistors are OFF. This happens when bright light is shining onto the photo-diode.

    A photo-diode is a current source. When small light shines on to the the photo-diode, the photo-diode saturates. When no light is applied the photo-diode is open circuit. The voltage across the photo-diode increases and this causes saturation of Q1 transistor. The Q1 transistor then switches ON the Q2 transistor and Q2 transistor switches ON the Q3 transistor that switches ON the bright LED.

    Calculating LED current:

    IledMax = (Vs - Vled) / Rd = (3 V - 2 V) / 100 = 1 V / 100 = 10 mA

    Rb3 is chosen for very low transistor gain.

    Ib3 = (Vs - Vbe3) / 1000 = 2.3 V / 1000 ohms = 2.3 mA

    Beta = Ic3 / Ib2 = Iled / Ib2 = 10 mA / 2.3 mA = 4.34782608696

    A typical transistor would have a current gain of at least 20 and this is at extreme temperatures or for transistors that are very old and have partially failed. Average current gain for a transistor should be about 100. Some transistors can have a current gain as high as 700. The current gain is influenced by the transistor:

    - collector emitter voltage,

    - temperature,

    - transistor type,

    - age,

    - production tolerances.

    Using a 1 kohm Rb3 resistor means that you can raise the output current by connecting another LED in parallel. However, 20 mA current (for two LEDs) will cause a 2 V voltage drop across the Rd resistor. Thus you should reduce it to just 50 ohms or 47 ohms.