Hello, I'm currently designing up a circuit to make an intervalometer for my camera so I can do some time lapse photography, basically a circuit that outputs high at a designated time period. I'm using a Schmitt trigger oscillator with various R and C values depending on what i want the time to be. The output I need to trigger a switch of some sort to trigger the camera via a remote cable i've made, just take a 3.5mm audio cable and to get the camera to trigger you have to connect the right and ground wires. I have a few SPST 5v relays laying around, but it seems the schmitt trigger only outputs about 3.4/3.5 vdc. So i'd probably have to get a 3v relay. But my question is, would using a transistor or other alternative to a relay be better for longevity? I've read a little on using a transistor as a switch, however I was a little confused on how it worked. Any help and/or suggestion are much appreciated. thanks in advance, Gavin
So on that would i just put the two leads from the remote trigger where the load is? And would that cause voltage from the circuit to flow through the remote trigger, because that's not what i want, I just need the two leads to touch, i don't want voltage traveling up to the camera. thanks for the timely response
You really do have a flow of voltage, though. (well, current flows but anyway...) +Vs is as you called it the "right wire" from the 3.5mm jack. Don't connect it to the timer's power supply. 0V is the "ground wire". It's also now a shared ground with the timer circuit. So do connect it to the timer's ground. Chip output is the output from your timer. RB depends on the gain of the transistor. If you use a BC337 (a decent all rounder NPN transistor), just chuck a 50 or 100 Ohm resistor in there.
So what I think I'll do is just use a relay, but use the transistor as a switch to turn the relay on. The output of the Schmitt isn't enough to trigger it, so I'll use the trasistor to switch it to allow 5v to flow to the relay. Now I hooked up the oscillator circuit and I can't seem to get it to work properly. I've got it all hooked up like every schematic you find out there for it, and it doesn't oscillate. After some troubleshooting, I found that it would oscillate until i put over like 3.2K ohms for R1. Is there like a maximum resistance for the chip or something that causes it to not work or something? So for example, for R1 i have 120K and for C1 I have 10uF. When I have that hooked up, it doesn't to anything, but if you do the math, it should come out to a 1 Hz pulse. Also when i plug it into Multisim, it oscillates but is kind of funky when I hook up the oscilloscope, the wave isn't consistent. So i feel like i'm doing something wrong.
If you want to use a relay for total decoupling, an opto-isolator is a far better method than a transistor nd a relay. If you're deadset on using a relay, don't forget your clamping diode too. What schematic are you using? A basic 50% duty cycle multivibrator? Because that would only need one resistor... By the way, are you using a Schmitt IC or an op-amp? Would using a 555 be out of the question? It's far easier and more reliable.
schmitt trigger, i believe it's a DM74LS14N and yes I can use a 555, but my professor suggested I try the schmitt oscillator because he thought that would be better, maybe a little more reliable. I was having some issues with the 555 frying on me for no reason. I hooked up the 555 oscillator circuit on a test breadboard, hooked it up to my camera, set it up to take pictures every like 2 seconds or whatever, walked away, came back about an hour later and it wasn't working. went through a couple chips that way. Quadruple checked the circuit too, it was all hooked up correctly. i haven't heard of opto-isolators before but just did a bit of reading. Would I use that in place of a transistor, or in place of the both transistor and relay?
In place of both the transistor and relay. You only need the transistor to drive the relay so it's redundant. The opto-isolator has much lower current requirements (you can think of it as an LED) so you can drive it directly. Honestly, I can't really think what the problem is. I was hoping when you mentioned changing R1 you were implying there was an R2 and had set up comparator circuit by mistake. I guess things can't be that easy, huh. What happens if you use alternate values such as 100uF and 12K to get the 1Hz oscillation?
yea, i think that's more or less my problem, I looked at the schematic and said, hey this will be really easy. so just because I said that, it won't work haha. Yea tonight I will hook up 100uF and 12k to see what it does. Then I'll just get another IC. But I tested that one just as a straight inverter and it worked properly. Also tried using a different schmitt on that IC, same deal. I used a 10k pot on it and at like 3K it was at what appeared to be like 7 or 8 hz and got progressively faster as I lowered the resistance. then when i got to about 3.2K it quit working. It's quite strange. that's what made me think that there might be some sort of max resistance or something? I would think plugging it into multisim it should work how it theoretically should. but it didn't. it would oscillate, but the time period wouldn't work right. i set it up to 1hz, and it would be HIGH for .5 seconds, which is correct, but then it would be LOW for like a second, then HIGH for .25 seconds.
Just out of interest, when the 555 fried, were you using the relay to trigger the camera? If so, were you using a clamping diode to protect from transient emf induced by the relay's coil switching off? That's the only reason I can think of for that circuit to go wrong.
Yes, and no. Although I wasn't using a transistor with the 555 circuit since it's out put was like 4.5/5v which is enough to trigger my 5v.
That'll be why it burned out. 555s can source about 200mA which is enough to switch a relay, but you need to include a protection diode in reverse parallel with the coil to protect the 555 from burning out.
ok cool. what does that exactly do? If you haven't gathered, I'm still at student haha i appreciate all the help you've given thus far
Well you've got more training than me, I've never even studied electronics. Yet. The long and the short of it is when the relay coil de-energises you get a voltage spike induced (transient current) because of the way inductors work. So you put a flyback/freewheeling/snubber/catch diode in reverse parallel with the inductive load. When a transient is induced it's dissipated by the diode rather than sensitive electronic components like the 555.
yeah pretty much what capn said, when the coil de-energises in the relay you get a massive back emf due to the magnetic flux lines being cut very quickly, the voltage can be anywhere from a few hundred to a few thousand volts which obviously fries the 555. Bang the diode in to block it. I have no idea why your professor recommended the schmitt oscillator its a pretty retro way of generating a steady pulse. The 555 method is far more accurate and stable, or you could use a crystal oscillator which is extremely accurate and not hard in teh slightest to setup.
