Simple light barrier – Part 3

In the past month I reworked the circuit of the receiver part of the light barrier and successfully tested it in a circuit simulator:


The circuit consist of the photodiode, a simple adjustable threshold trigger and a booster stage. The threshold trigger consists of a potentiometer, which is used for adjusting the threshold current of the trigger and a p-Channel MOSFET triggering a signal when the current which flows through the photodiode is lower than the threshold value. In this case the MOSFET is in conducting mode and the current flowing through gate and source will be boosted to a maximum by the booster stage which is marked by the red border. In the booster stage the two resistors should have a high resistance, but the resistance should be low enough to bring the transistors in saturation mode. In my simulation a value of 100 Ohm by 12 Volt input voltage was a good resistance value. The higher the resistances and current amplification by the transistors are, the more you will get a On-Off-Behaviour on the output. The customer is connected on the two clamps of Vout and could, for example, be a MOSFET controlling the power supply of some device.

Simple light barrier – Part 2


Simple light barrier – Part 1

For a project I’m currently working on I need a light barrier. For maximum efficiency the light barrier should have a high resistance in idle state and in triggered state it should have a low resistance, because it will be most time in idle state. There were some building kits for light barriers in my favourite electronic shop, but they need too much space for my project so I need something more compact. Another point is that they use relays for switching which are too sluggish for my application, need too much electrical power and are too expensive for me because I need many of them. So I decided to design it own my own:

Lichtschranke (bei Dunkelheit leitend)

In Idle state, the light, emitted by the LED is being catched by the photo diode (can also be a photo transistor if necessary), so a high current flows through the photo diode and the resistor Rn. This results in a positive gate-source voltage of the p-chanel MOSFET, so it locks the current flow through the consumer Rv. For reducing enegy loss, the resistance Rn should be as high as possible. In triggered state, nearly no current flows through the photo diode, so the gate-source voltage of the MOSFET is negative, so the MOSFET will turn on and let the current flow through the consumer Rv. The resistance Rb is just a base resistance for protecting the LED from overvoltage.

I will build a prototype of this circuit at the end of this month and will post my test results on this blog.

Simple light barrier – Part 2