Need help testing this circuit

[Mark34]

Someone from another forum told me ''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"

Its true?

 

[KMoffett]

How about a schematic?

Ken

 

[Mark34]

Page one first post.

 

[Audioguru]

If you want the SCR to be triggered when the light is blocked then what you were told is wrong because the person wrongly thought you wanted the SCR to be triggered when the phototransistor has light.
When the phototransistor is lighted then it conducts and turns off the 2N2222 transistor. When the light is blocked then the phototransistor does nothing then the base resistor turns on the transistor which triggers the SCR.

 

[Mark34]

Its what I said I need the SCR to trigger and turn on a light led when the photogate light is blocked, so is the phototransistor correctly placed in the circuit and the light led should be conected between SCR anode and supply's +9V ?

 

[Audioguru]

Its what I said I need the SCR to trigger and turn on a light led when the photogate light is blocked, so is the phototransistor correctly placed in the circuit and the light led should be conected between SCR anode and supply's +9V ?

Your SCR has no part number so we don't know if the 10k resistor will feed its gate with enough current and we don't know if the LED will draw enough current for it to stay turned on.
The LED and/or SCR will make a spectacular explosion if you do not limit its current with a series resistor.

 

[Mark34]

SCR is BT 151 .

 

[Audioguru]

It is a very powerful SCR that needs a lot of gate and holding current:
1) Max gate current needed= 15mA then ... the circuit should be re-designed.
2) Max latching current needed= 40mA that is higher than the max allowed current of an ordinary LED.

Why not use a little 2N5061 SCR? Its max voltage is 60V, max load current is about 0.5A, max gate current is 0.2mA, and its max holding current is 5mA.

 

[Mark34]

Ok thanks, I will get a lower voltage SCR and a real phototransistor..

 

[Audioguru]

On the other website you showed a photo of your photo-receiver that is not a phototransistor and I think you had its pins mixed up. It rejects continuous light. It needs bursts of 38kHz modulated IR radiation. Maybe your LED produces visible light but not IR radiation? And your LED does not produce bursts of 38kHz?
Like I said on the other website, you do not need a lower voltage SCR, you need a much lower current SCR than the monster that you have.

 

[Mark34]

Is a Mcr 22-6 SCR good for my circuit ?

 

[Audioguru]

The Mcr 22-6 SCR is rated for 400V and 1.5A but it has a sensitive gate and low holding current so it should be fine.

 

[duke37]

I have some C106D SCRs, old but mature. These will also do.

 

[Mark34]

I replaced the SCR and phototransistor and test light led stays turned on all the time no matter if the photogate is blocked or not, voltage between ground and base transistor 2N2222 is 4,5V with photogate blocked and 2V with photogate clear.

 

[BobK]

Did you adjust the pot? Those voltages should change if you do so.

Bob

 

[hevans1944]

I replaced the SCR and phototransistor and test light led stays turned on all the time no matter if the photogate is blocked or not, voltage between ground and base transistor 2N2222 is 4,5V with photogate blocked and 2V with photogate clear.

Sounds about right. Once it is triggered "on," an SCR will stay "on" until the current through it drops below its holding current or the circuit is opened. With DC applied, this cannot happen unless you open the circuit.

I don't suppose you actually read my post #8, where I suggested you use a low-voltage transformer winding to power up the test light LED, instead of using your 9 V DC supply. Or where I suggested inserting a normally-closed push-button switch in series with the SCR to turn it off after it latches. Mo betta, mo simpler than the transformer approach, but it does require manual intervention to "reset" the SCR while you try to find the proper "sensitivity" setting for the variable resistor.

The 2V between base and ground on the 2N2222 sounds a little high with the photogate clear. Suggest that you Increase the resistance of the variable resistor to make the voltage lower, approaching 0 V with the photo-transistor (PT) illuminated. If the variable resistor is already at maximum resistance, insert another 100 kΩ in series with the variable resistor. Wash, rinse, and repeat until you find a value of resistance that will NOT trigger the SCR with the photogate clear. Then verify that the SCR will latch on when you block the light to the photogate, causing the base-to-ground voltage of the 2N2222 to increase towards +9 V.

 

[Mark34]

I will try that hevans1944, is it normal that the voltage at the testing light led stays always at 2,8V regardless if the photogate is blocked or not.

 

[Audioguru]

Maybe you do not know how an SCR works. After it is triggered then it stays turned on until its power supply is turned off. If you want the test LED to turn on and off when the light is blocked and not blocked then you need a transistor or Mosfet to drive the test LED, not an SCR. It was suggested to power the LED with AC that turns off the SCR every cycle of the AC.

 

[hevans1944]

I will try that hevans1944, is it normal that the voltage at the testing light led stays always at 2,8V regardless if the photogate is blocked or not.

What do you mean by "voltage at the testing light led?" Voltage is always measured between two points. If one of the two points is the anode of the led and the other point is the cathode of the led, then it is normal for there to be a voltage measured between those two points if the LED is conducting current.

If the LED is conducting current in the forward direction, anode positive with respect to cathode, then it should illuminate: red, yellow, orange, green, blue, or white. The color of the illumination determines what voltage will appear between the LED terminals. Infrared emitting LEDs have the lowest forward voltage. White-light LEDs have the highest forward voltage. If the SCR is always "on" there will always be a voltage across the testing light led.

It was suggested to power the LED with AC that turns off the SCR every cycle of the AC.

I suggested this for test purposes to allow easy adjustment of the "sensitivity" variable resistor. On the high-speed photography website, they mention several different means of illuminating the photo-transistor (PT), such as with an LED, a laser, or a flashlight beam at different distances from the PT.

In one example, it was suggested to aim the "beam" across the top of a pop-corn kernel on a hot plate and adjust the sensitivity until the SCR was ready to trigger when the light path to the PT was interrupted by the kernel exploding.

The SCR, when it is triggered into conduction, acts like the sync contact on a camera's hot shoe, closing a circuit and firing a flash, effectively "freezing" the motion in the picture at that one moment in time. If you use a DC source to power the test light led, then you need a normally-closed push-button switch to interrupt power to the SCR and cause it to reset to the non-conducting state. Of course the PT must be illuminated before pressing the reset switch or the SCR will immediately trigger again.

The problem is getting the "sensitivity" adjusted "just right" before (1) resetting the SCR and (2) opening the camera shutter to take a picture when the light to the PT is interrupted. Trial and error is the easiest way to make this adjustment. With AC excitation of the test light led, you can adjust the sensitivity until the led flashes "on" when light to the PT is interrupted and stays off when the light path is clear. After getting the sensitivity adjusted properly, you can connect the SCR terminals to the flash unit that you want to trigger.

This is all assuming that the OP wants to use the circuit to take photographs illuminated with a flash unit triggered by something interrupting the light path to the PT. But who knows WTF the end game is... could just as well be an intrusion detector FAWK.

 

[Audioguru]

In a conversation, Mark said that he wants blocking and unblocking the light from the phototransistor to turn the load LED on and off and if replacing the SCR with a transistor will do it (like I explained in my last post here) and I said yes, but I also explained all the missing details that are important to do the actual design.