A simple circuit for detecting when a composite video signal stops? - Page 1
Forum section: electronics / beginners

author: Orihalcon
author_rank: Contributor
timestamp: October 19, 2024, 02:27:11 am
content: Hello all,I am trying to come up with a simple way to trigger an automation system when a VHS tape's recorded content finishes playing.I have verified that the VCRs I'd be using do not "blue screen" on unrecorded tape and that there is no sync signal output for unrecorded tape.My thought is that an LM1881N sync separator could be used to strip the composite signal down to just sync pulses. Then, another IC could detect the presence (or absence) of those sync pulses and report an "on" or "off" state as a closed or open switch which would be used by the rest of the automation system.Can anyone recommend a simple circuit on how I'd actually wire that along with another IC (no idea what IC or circuit I should pair this to) that can open or close a switch when it detects or doesn't detect sync pulses after a few seconds?I'm open to other ideas as well if there's something simpler.It'd be really cool if it could trigger based on when the "tape counter" stops moving instead of the composite video signal going away because this could also be used for VCRs that do "blue screen" on unrecorded tape which is the case for most DV and 8mm format players. Those blue screens do have sync pulses, so I don't think my above method can work with those players to detect when recorded content is over.Thanks in advance!

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: October 19, 2024, 04:47:12 am
content: Hello Orihalcon, the LM1881 is the usual way to strip sync pulses from analog video. Connecting its sync output to a 74HC4538 monostable one-shot FF will get you a video presence/absence detector. Program the one-shot for greater than one TV line (usually 100 - 300 uSec) and you have it. From the '4538 you will get both polarities, that is, presence = H and presence = L from its two output pins. DigiKey has tons of them in stock for cheap. The 74HC4538 version is TTL compatible (+5V only) and the CD4538 CMOS version will run off anywhere from 5V to 12V in case that's what you have. Those 4538 outputs can then drive a transistor base to generate closure-to-ground outputs or relay closure.

author: Orihalcon
author_rank: Contributor
timestamp: October 20, 2024, 12:53:51 pm
content: I was thinking about some other possible solutions as well that may be simpler than a 74HC4538 (since looking at the data sheet for if makes it at least appear pretty complicated - I still ordered some of those to play around with).Since I don't need to know the exact instant that composite video is gone (just need to know within a few seconds if composite video is gone or comes back), would it possible to use the Csync pulses themselves in combination with a diode to charge a small capacitor and a bleed resistor across the capacitor so that when sync pulses go away that the capacitor voltage drops? The idea is that a second component like a voltage comparator, op amp, or maybe a certain type of mosfet that will change states to effectively an open or closed switch based on what the value of the voltage stored in that capacitor is.I've attached the typical outputs of the LM1881 diagram. I believe that the sync pulses it gives are at TTL levels, so they should be around 5V which is much bigger than the original Composite sync pulses which are only around 0.3V.My problem is that I have relatively little electronics knowledge to design this circuit. I could probably play around with some combination of these components and an oscilloscope to get it working, but if anyone can suggest specific components and a circuit diagram, that would save me a lot of time and frustration haha. I also don't really know when decoupling capacitors and other items make sense to use etc.

author: Ian.M
author_rank: Super Contributor
timestamp: October 20, 2024, 01:34:06 pm
content: Yes, you could capacitor couple the composite sync to a couple of diodes to make a charge pump, like this. (attached)C1 determines how much charge gets pumped through the diodes per pulse - as the LM1881 outputs are only good for a few mA, and it must fully charge in the interval between edges, don't make it too big. C2 accumulates the charge pulses pumped through the diodes, limiting at the peak to peak voltage of the input less two diode drops. R1 discharges C2 slowly. Choose it so that the output voltage doesn't drop more than 20% between sync pulses. When the sync goes away for a frame or more the output will drop to near zero. The output is analog so square it up with a Schmidt trigger or comparator before using it to drive digital circuits.