Close

Turn ON In The Dark Light

diy-electronicsDIY Electronics wrote 03/29/2020 at 10:28 • 3 min read • Like

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:

https://hackaday.io/page/6937-light-control-system

https://hackaday.io/page/6987-opamp-instrumentation-tool

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.

Like

Discussions