IR Sensor Inverted - IR rx equals light off

542

06:32:53

The transistor with the free leg is actually a photo transistor. The first led in circuit is IR but there's no adjustments for that nm range.So I'm forgetful and I keep leaving the house forgetting to latch the door in the other room. This is designed to put the status led at the main door and sense the reflection of IR on the latch of the second door. If it's unlocked I get a warning light as I'm leaving. It's in the breadboard now and working very well. I kept upping the resistance until I got the desired behavior but I am uncertain if the values are sound and not overdriving anything. The simulator doesn't help because I have no way to SIM the photo transistor. Any comments or calculations greatly appreciated.

published 10 years ago

Sine_eyed

10 years ago

It's a cool idea in theory, but I can see there will be a couple issues in design and testing. First, as you've already found out it's impossible to simulate it exactly as we're lacking a crucial component. However you could use a standard transistor with a potentiometer tied to it's base as a facsimile to the phototransistor. High resistance when no light detected, low resistance when the IR beam falls on the photran..

Sine_eyed

10 years ago

I have a couple of alternatives to using this particular circuit of you're interested- but if you've made up your mind and want to do it this way, I'll try to help you there too..

nikki2beeps

10 years ago

Cool idea,I didn't think of pot to simulate photo transistor. I'm guessing I should get resistance values for photo transistor when ir is present or not? Fyi photo transistor was salvaged from old vcr so no data sheet, regular transistor is 2n3904, chosen for availability. Free is very important right now.

Sine_eyed

10 years ago

No, don't bother with resistance values of the phototransistor. Say, do you still have that old VCR? It'll have a ton of stuff we could use for your project..

nikki2beeps

10 years ago

Yea I do, I've broken all the mechanical down and pulled the ir tx and rx pair, actually it was 3 rx with a neat prism to verify tape is in place and front cover is open, plus one was being shot through a hear with curious in it for timing measurement.

nikki2beeps

10 years ago

For what it's worth the current configuration is in place and working very smoothly. The 14k resistor is enough that daylight doesn't interfere but the decreased sensitivity means it has to be mounted within 1/2 in. of the latch. I'd like a higher sensitivity but that would require looking for a specific freq. or pulse.

Sine_eyed

10 years ago

Lost track of ya for a couple days, sorry. Well good job getting it installed and operational! But yeah, there's really no need to use a phototransistor at such a close range- an LDR can do the same job, just as reliably. I know its cool to make circuits like this, but sometimes a simpler (less cool) solution can be even more effective. You might consider using a contact switch in a open wire type security circuit. Affix one contact to the latch and another where the latch makes 'contact' with the wall, effectively closing the circuit and allowing current to flow through the LED. You wanted cheap? It doesn't get much cheaper than that!.. ;^)

nikki2beeps

10 years ago

I'm not familiar with an ldr? I guess I should have made it clearer but the latch is on a garage door so having part of the circuit on the door itself was not a viable option. The ir allowed me to mount the circuit on the wall without having actual contact with the latch. The alignment of a physical switch was not desirable due to seasonal temp changes . The only other way I thought of was a magnetic reed switch. IR was free.

Sine_eyed

10 years ago

LDR= light dependant resistor. Also called photoresistor, or photocell. They're really simple to use but, ugh, is hard to say what might be best without knowing dimensions, spacing, obstacles- all sorts of stuff. One thing I've learned about electronics is that a seemingly simple project can turn out to be anything but! But don't be discouraged- well find a solution. Some time has passed- have you done any more tests, or have you tried to install it?..

ankitmhatre

10 years ago

Yes Sine_eyed sometimes its most important parameter becomes the dimension

nikki2beeps

10 years ago

Oh yes, it's installed and running well. As the garage latch closes it reflects the ir back to the receiver and the indicator led that's mounted at the main door turns off. Had to paint the latch white on the backside but it is working and daylight hasn't caused any ir interference.

Sine_eyed

10 years ago

Well good! Glad to hear it.. =^)

yainalaikhok

7 years ago

889

yainalaikhok

7 years ago

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