| ??? 03/22/08 22:23 Read: times |
#152450 - Yes, it's clearer, but ... Responding to: ???'s previous message |
Asok Sankar Rudra said:
Hello Everybody, Richard Erlacher said Could you clarify how you apply the relay, please? If it is driven by a switch, then you could probably omit the relay, too, again, provided it doesn't violate some regulatory restriction. It's possible you might need either the relay or the optoisolator because you must be isolated from the signal source, but you probably don't need both, as either one provides isolation. Thanks Richard, 1. There are 10 IR remote control handsets in 10 different rooms numbered 1 to 10. How does the IR figure into the system? Are these handsets connected directly to the system you're building? How long are the cables to/from each room to the system you're building? What sort of cable are you using? 2.Each of these 10 rooms have wall mounted receivers suitable for above handsets. These are IR receivers, are they not? 3.Each of these 10 receivers,when activated by a key press in the handset,toggles the relay inside the receiver.
When you say "toggles", what, exactly, to you mean. Toggle normally implies a single state-change. I'm interpreting this to be a momentary contact closure, which would, by its momentary nature, be two state changes. That's essentially a single pulse, once it's debounced. What is the duration of that contact-closure? 4.Each of these 10 receiver-relay's contact points (two in numbers) are in turn taken out of the rooms to be connected to a remotely located central-detector. Your system is the "remotely located central-detector" if I understand you correctly, am I wrong about this? 5.This central-detector has to detect which all room's remote receiver has been activated and naturally take programmed actions.
Kindly let me know if the picture is clearer. With best regards, A.S.Rudra It seems to me that you really have no need at all for electrical isolation. Apparently the 12-volt supply is a given over which you have no control. I would suggest you simply attenuate the voltage on these two wires as previously described, and buffer them via a 40106 hex Schmidt-trigger inverter. Since there are, conveniently, ten inputs, I'd suggest you consider a 74HC147 priority encoder IC if you don't want to encode the ten inputs in firmware. Since you're cost-sensitive, and have plenty of time, as it's a human-activated system, I'd suggest you actually encode the inputs in your MCU. Now, we've eliminated the need for optoisolators, and we've not got to build in any additional relays, since there's no need for isolation. Since the cables are likely to be quite long, I'd recommend you use a diode (anode to GND in parallel with the voltage-divider) to limit the excursion of the voltage across the input pair. I'd also recommend a clamping diode to Vcc at the positive input to the Schmidt-trigger inverter. The current on the cable may exceed the maximal clamp current in the CMOS Schmidt-trigger inverter's input, and this diode clamp will protect that IC, provided you choose a schottky diode having a forward voltage less than 0.5 volts. That current should, based on the resistor values previously mentioned, always be less than 6 mA so a small signal schottky diode (the cheapest available) will probably work just fine. Be sure to use a capacitor on the input of the CMOS Schmidt-trigger that completely swallows the relay-contact-bounce and filters out the majority (in amplitude) of the noise, but doesn't swallow the entire pulse from the relay closure. RE |
