Let's Make Robots!

Using a Thyristor and IR Sensor?

Hi am building this circuit.

I am using a VS1838B Infrared Sensor. On the right is a BC327-25 PNP Transistor and on the left is a MAC97A6 Thyristor. It seems to work well upon first use. then when you turn the battery case off and then back on again the thyristor is still turned on. If I pull one of the wires out and then quickly push it back in it will work again with the IR remote turning on the light and it stays on. but when I turn the battery pack on and off to reset it the Thyristor it doesn't reset or maybe the on off switch causes a spike or interference so it comes back on. Any suggestions on how to make it so the LED comes on when i put my remote control to it and then turns of and stays of when I turn the battery pack on and off.

I tried the resistor from Gate to Ground and that worked to prevent it from turning on whenever I switched the battery on and off however now it seems like it doesn't have enough juice to turn the thyristor on. Any ideas? Any suggestion on better components or different Thyristor?

Thanks for the help!

Zyan Mayfield

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Thanks thats useful information. I didn't know about Triac and SCR. I'm going to be getting some 2N5060 SCR Thyristors and trying it. Yah my IR Sensor is a 38khz IR Reciever. Any more advice other than trying an SCR Thyristor? My main goal is to make a robot then has wire remote can move around the room uses dpdt switches to switch pollarity on each motor and momentary button to go for each motor. I want to be able to drive it around the room and sense multiple things. When it senses IR the light will turn on and stay on. When it senses a dark room the light will turn on and stay on as well that way when they come back to the finish line it should have sensed these different things. It's for a elementary robots class! Thanks for the help thus far! I Have everything working except the IR sensor.



PS Here is the Data Sheets for future reference.

PNP Transistor - http://pdf1.alldatasheet.com/datasheet-pdf/view/50727/FAIRCHILD/BC32725.html

IR Reciever - http://www.hyzt.com/manager/upimg/200987165540.pdf (Sorry could only find it in chinese.)

Triac Thyristor - http://www.nxp.com/documents/data_sheet/MAC97A8_A6.pdf

SCR Thyristor - http://www.onsemi.com/pub_link/Collateral/2N5060-D.PDF


Also, your "thyristor" is a triac.  Normally, a triac is used in ac circuits.  An SCR is more appropriate for DC.  I had to Google for datasheets on all three components.  Would be nice/helpful if you at least included links to the info.

Well, you definitely need a resistor in series with the LED.  Without a resistor or something to limit the current an LED won't last long.  Oddbot did a great tutorial here on calculating resistors for LEDs.  You can find it using the search box.  You also need a pullup on the output of the IR sensor.  You also will most likely need a couple resistors around the transistor.  Also, the voltage is too low.  The IR sensor needs at least 4.5 volts to work properly.  It is specified for 5V.  Also, all the other components: LED, transistor, and SCR or whatever, will have various voltage drops.  Each will be typically 0.5 to 1.5 volts.  They add up quick to give you 0volts depending on the circuit.  As Birdmun pointed out, the IR sensor is designed for 38 Khz remote control.  With your circuit all you can hope for is turning it on whenever ANY signal is received, noise or valid.  There will be no way to turn it off.  ANY output from the sensor will trigger it.  The only way to turn it off is cut power.  With a schematic and more details I or someone else can give better info.


Sorry about not having an actual schematic. When I get back home I may convert the fritzing to a schematic. But no it's not a large capacitor that has 3 pins, It is the IR Sensor. I'm trying to figure out where I would benefit to have resistors. Every where I have tried keeps it from turning on when the battery spikes through the on/off switch but doesn't allow enough when using the remot to actually switch the thyristor.



You should post a schematic of your circuit.  It is nearly impossible to make sense out of the picture you posted.  A schematic gives much more relevant information.

Having said that, here are a few notes.  First, you probably need some resistors in there. Second, the image of what I can only assume is your capacitor looks huge.  If you have a large value capacitor the circuit is going to have a very hard time charging it.  Imagine a two year old child trying to fill a swimming pool with a bucket.  Third, you say you have an IR sensor, but the only component that could reasonably be considered it appears to be an LED.  Fourth, the circuit is going to be EXTREMELY sensitive to noise:  circuit noise and IR noise.

An actual schematic with the actual parts used listed and the pins properly labeled will help a lot.

his IR sensor. It rather looks more like a 38khz IR receiver. Without a microcontroller his IR receiver won't do much that is useful to my knowledge.

You are right, of course.  It can be used to turn the triac on whenever it receives a signal.  That's about it  Any signal, no matter what it is that causes the output to transition even for a very brief moment will turn the triac on.  And since it is a triac (or better an SCR) it will stay on until current is removed.