| Electronics > Beginners |
| VS1838B (TSOP4838) for a beam break IR detector. |
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| Rigolon:
I'm trying to design a mid range barrier (about 10 pairs or more of emitter receiver) beam break IR detector. (2 to 3 meter range) In my country I don't find many varieties of components, at least not as a hobbyist. The best emitter that I found was the TSAL6200, but I would have to drive at 1A to get a good mW/sr (about 600mW/sr). Since I'm planning on use several emitters I'd like to avoid using this much power. In my experiments the modules such as VS1838B are more sensitive than regular photodiodes and phototransistors. (The best phototransistor I found was the PTC334-6C and per the datasheet I would need Ee = 1mW/cm² therefore a 600mW/sr emitter would work at a maximum 25cm, if I get the theory right Ee = Radiant Intensity(mW/sr) / Distance²) My thoughts were to use a uC PWM and a regular transistor to drive the emitter at 38kHZ with 100mA and direct it at a VS1838B so the output goes LOW. I don't know the emitter I'm using because I got it from an old TV remote control. But here is what happened: The only time my output went LOW was when I started the emitter (it would be LOW for half a second) and if I broke the beam it would go LOW very briefly (just a blink) after I stopped breaking the beam. Shouldn't the output be LOW as long as the receiver is irradiated with 38kHz? And only go HIGH when I break the beam? Or perhaps I'm doing something wrong? PS.: I'm going to buy some FSH 4550 (good radiant intensity with 100mA) from aliexpress but it will take a lot of time and I don't want to pause this project. |
| Buriedcode:
The modules have quite a bit of circuitry in them including an AGC (amplitude gain control) amplifier. This allows it to be very sensitive and pick up remote control flashes bouncing off walls, whilst not being "blinded" by someone using the remote up close. They also have bandwidth and expect a changing signal, not just modulated at 38kHz but also pulses, which is what remotes use. They aren't designed to pick up constant 38kHz carrier. To get around this you would have to double modulate - modulate the emitter at 38kHz, but also modulate this at say 1kHz. This is trivial for a microcontroller to do, you really just create say 30 pulses with on and off times of 1/38/2 = 13.2us. |
| SeanB:
To get the current down in the transmitter you run the IR emitters at 1A each, and as you are going to use a few of them put them in series, and use a power MOSFET to drive them, with a large value ( 1000uf low ESR) capacitor in a small area close to the emitters. Drive with very narrow pules, around 1us each, and you will have a low average current, but high peak current and thus high output. As above you need to have both the 38kHz modulation and the lower frequency modulation to get the receiver to function, so use a microcontroller to make it, or just have 2 oscillators using CMOS gates making them, and use an or gate to get the final signal. Receive wise you just are looking for lost pulses, so either a microcontroller input and it resets a timer on the edges, and if the timer overflows because of no signal you have an output, or just use the signal to drive an AC coupled emitter follower amplifier, which has as emitter load a RC that will feed a comparator, RC being such that the input being absent for 3 cycles (or more) means the comparator changes output state. |
| Audioguru:
The datasheet for a TSOP IR receiver explains that it reduces its sensitivity when it receives steady IR from light and when it receives steady 38kHz from a compact fluorescent light bulb. The datasheet talks about its burst of 38kHz length and required gaps in its reception of pulsed data for it to have high sensitivity. Maybe you can design your beam to have the required bursts and gaps of 38kHz. |
| Rigolon:
Thx for all the responses. Here is my idea for now: I will use a 556; 556a output at 120Hz with 10%D.C. to switch a transistor (Q1), the emitter of Q1 goes to the collector of Q2 (another transistor) that is switched at 38kHz. The IR Emitters will go on the collector of Q1. 100mA on the emitter was more than enough for the distance I wanted. P.S. I may use an ATtiny85 instead of the 556 to get these two PWM An important detail, for those who don't know, is that when the VS1838B doesn't have IR light on it the output is HIGH This way I was able to get a LOW on the VS1838B with 120Hz Signal. What I have to do now is to keep the output always LOW. So far I've come up with two ideas. 1) Since I'm using a uC, the first one is to use the timer on the uC to create a frequency counter and if the frequency measured is different than ~120Hz I'll know that there is something blocking the beam. I want to use several VS1838B, about 15 spaced of 3 or 5 cm, so I'll have very few space where it will not detect the presence of the object, but since the objects will have more than 5cm I'm ok with that. My problem with this solution is that, since I'm going to use several IR Receivers, if I use them in parallel I would have to block all beams for my uC doesn't see the 120Hz signal. And I'm not sure if I can use them in series. (I only have one with me, already bought some more, but takes some time to arrive). 2) The second idea is to use a small signal transistor after each output to invert my signal. So now if my emitter/receiver is unblocked I should get HIGH output at 120Hz and use a Capacitor to eliminate the AC component and get a constant HIGH signal, if the beam is broken the output goes LOW. My problem with this solution is that I would have to wait for the capacitor to discharge to get the LOW signal and I'm afraid that the RC time to keep HIGH all the time it's going to make too slow to get to LOW, I haven't done theses calculations yet, will do when I have a bit more time. If you guys have any suggestion I would appreciate it. Thanks |
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