Transistor-triggered photogate

Thread Starter

Mark34

Joined Sep 23, 2016
12
Hello !
I build this Transistor-triggered photogate circuit from a website and it doesn't seem to work. When the light between the emitter and detector diodes is interrupted it should trigger the SCR and a turn on a led, can someone tell me where to connect a light led in order to test it ?
http://hiviz.com/tools/triggers/triggers3.htm
PLEASE i need first the modified schematic posted here and after maybe some explications, and be sure about it .

 

DickCappels

Joined Aug 21, 2008
6,964
The phototransistor is connected in the wrong place. As shown the gate drive is reduced when light hits the phototransistor.

To correct swap the phototransistor with the 100k pot and put a 1k resistor in series with the emitter of the photo transistor such that the phtototransistor's collector is connected to +9 volts,the emitter connects to a 1K resistor (approx value,) the other end of which connects to the base of the 2N2222.

You will need to use a sensitive gate SCR -one that only needs a milliamp or two to fire, even at that you will probably have to reduce the value of the 10k resistor in order to get enough gate drive current.

To test with and LED connect an LED with series current limiting resistor from +9V to the anode of the SCR. Te resistor in series with the LED must limit the current to greater than the holding current (IH) of the SCR or the SCR will not latch on.
 

Thread Starter

Mark34

Joined Sep 23, 2016
12
Thanks for the instructions, is there any problem if instead the phototransistor I use receiver diode ?
 

ScottWang

Joined Aug 23, 2012
6,988
Do you have use any black pipe about 2 cm long to cover the photo-transistor when the paper or card to blocked the light get into the photo-transistor, does it has any light from environment to affecting the photo-transistor?

You can use the volt-meter to measures the Base of 2N2222 when the light get into photo-transistor and the card blocked the light, how many volts and do they have any difference?
 

Thread Starter

Mark34

Joined Sep 23, 2016
12
Ok I'll do that, problem is I am not sure which one is the emitter and collector for the phototransistor, I posted a picture how I think it is.
 

be80be

Joined Jul 5, 2008
2,051
You need to no for sure what you have?

If you have one of them it's not a phototransistor.
It doesn't like 9 volts and they don't last long hooked up wrong.

Phototransistor normally have only 2 pins, And the one's with 3 pin's don't
look like the above there like this.

some have a clear look like this

and come in black case too but there thin case.

This is your more normal phototransistor
years ago they had metal cans.

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Last edited:

Thread Starter

Mark34

Joined Sep 23, 2016
12
Mine looks exactly like this


Can i replace the phototransistor with a photodiode for my circuit ?
*The last picture you posted is a photodiode.
 
Last edited:

be80be

Joined Jul 5, 2008
2,051
You can use a photodiode but they do not work not the same.
you have to change things a bit


A photodiode is a semiconductor device that converts light into an electrical current. The current is generated when photons are absorbed in the photodiode. Photodiodes may contain optical filters, built-in lenses, and may have large or small surface areas. Photodiodes usually have a slower response time as their surface area increases. The common, traditional solar cell used to generate electric solar power is a large area photodiode.
 
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