Title of the circuit is deliberately given because it is used for testing the Photo diodes, Infrared LEDs, Photo transistors etc working in “Silent Mode”. How can we confirm an Infrared LED is working or not. It gives No visible indication. It is working and emitting Infrared rays but the IR rays are beyond our spectral response and we can’t see it. So this simple tester will help you to check whether such devices are good or bad before using in a circuit or even during trouble shooting. Not only one function the tester doing . It can test, all diodes, Capacitors, LEDs etc. Its cost is just Rs.10 but it is a worth tool.
These are the common types of IR LED, Phototransistor, Photodiodes etc now commonly available. See, all are alike and very difficult to identify visually whether one is Transmitter or Receiver. So this tester will make identification cum testing very easily.
To make the tester, you need the following:
1. A small piece of Dot type Common PCB
2. Three resistors – 1K, 10K and 100 Ohms
3. One BC 547 NPN transistor
4. A Bicolour LED – Red and Green – Common Anode type
5. A connector pin or probes as test points
6. A 9 V battery snap
The Bicolour LED is used for the visible indication of the invisible working of IR LED. A common Anode type Bicolour LED is required. It has two chips –Red and Green– inside with separate Cathodes. Anode is common for both the LEDs. You can easily identify the leads. The longest middle lead is Anode. A long lead is the Cathode of Red Half and the short lead is the Cathode of Green half.
See image. It is too simple. Test point is marked as A and K. Point A is for connecting the Anode of the IR LED and K for its Cathode. Base of the BC 547 transistor is connected to the K point with a 10 K resistor going to ground. Common Anode of the Bi colour LED is connected to the Positive rail through the 100 Ohms resistor. Note the Cathode connections. Cathode of Red half is connected to the Collector of T1 while the Cathode of Green half is connected to ground directly.
When the circuit is connected to the Battery, Bicolour LED lights as Green because, it’s Green half gets direct current path from positive to negative through the 100 ohms resistor. So Green colour indicates that the tester is ok. Now what about the Transistor T1 and Red half of Bicolour LED? The base of T1 is floating because there is no IR LED connected to the test point. So T1 will be off and the Red half of Bicolour LED remains dark because it has no current path.
When an Infrared LED (Tx) is connected with Anode to point A and Cathode to point K, current flows to the base of T1 and it conducts. Bicolour LED turns Red because it gets current path through T1. At the same time, the Green LED turns off because, most of the current passes through the Red LED and T1. So if the bicolour LED turns Green to Red when the IR LED is connected, it indicates that, the IR LED is forward biasing and good. If the Bicolour LED is not changing Green to Red, it indicates that the IR LED is bad and not forward biasing.
This test is applicable to all diodes and LEDs with Anode in point A and Cathode in point K.
But for IR Receiver LED (Rx), Photo diodes, Phototransistor etc, the leads should be reversed because, these devises show “Reverse biasing” like the Zener diodes. So connect Cathode to point A and Anode to point K.
See the test procedure given below.
For Capacitor testing, connect the +ve pin to point A and – ve pin to point K. Capacitor charges quickly and the colour of Bicolour LED gradually turns from Green to Orange and then to Red. This change depends on the value of the capacitor. For example, 1 uF capacitor has short duration for the colour change and a 1000 uF capacitor takes long time for the colour change. If this happens, the capacitor is good.