remote control fan heater

[Jeff J]

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Because my Autosleeper has no heated rear screen I have fitted a 12v fan heater to de-mist it. However I cannot get a wire from the dash to rear so have to go into the back to switch it off after use. I am trying to build a remote switching device from recycled components but because I only have basic knowledge of electronics I am not 100% sure about my cct. design. Are there any experts that could look at my drawing & give any advice. Thanks.

[ghost123uk]

Much as I admire your circuit design and dedication, you can buy a key fob style remote controlled relay off ebay for about £2.99 (I use one on my Propex)

[Jeff J]

Much as I admire your circuit design and dedication, you can buy a key fob style remote controlled relay off ebay for about £2.99 (I use one on my Propex)

Thanks for the tip , when I looked they were about £15 & I already have all the components recycled from another device, I''ll have another look. Do you know if the cct will work?

[ghost123uk]

Well firstly I am not sure what a "cct" is

But I have looked at the schematic

I do not know what type of output that receiver gives.
Is it "latching" or "momentary" ?
What current can it supply ?

I never liked working with CMOS IC's

I think if I were doing it, depending on the answers to the above questions, I would try a simpler approach. I would fire the TIP41C either directly off the 1.5 volt data out put of the receiver (assuming latching) via a current limiting resistor and a cap for smoothing, or if necessary via a simple 100mA npn "slave" tranny (BC182L perhaps).

However, you might well be ahead of me here as I guess there is a good reason for you to include the flip flop, is that because it is a non latching output on the receiver ? (if yes, I would be looking at utilising an SCR somehow, but that's just cause I don't like messing with the added complication of CMOS )

[Mocki]

You are aware that there is a very easy way to run that cable to the rear? Up the drivers door (A) pillar and along the ducting to the back corner, the way the interior light cables run?
I used a length of stainless steel antenna to go down the door pillar from the interior light hole (after dropping the interior light out) and pulled the control cable back up, and then used the same method between the other interior light holes to the back.

[Jeff J]

Thanks Mokki, yes I have read in other posts about getting a cable along that route, but I am thinking that if I can do it this way I could also create an immobiliser along similar lines.

Sorry Ghost, cct. is my abbreviation for circuit.

This has become a bit of an enjoyable learning project as well as a practical gadget.

I think the receiver gives a momentary output, ie it only gives an output while the button on the fob (transmitter) is activated. I do not think it will handle much current as it is such a tiny thing(not very technical, I know )

I have just been looking up the way the components work on data sheets & wikipedia & trying to utilise them to suit my needs. As I said I only have a very limited knowledge of electronics & I will have to look up what an SCR is & does & if I have one.
The 200 ohm resistor is supposed to be the current limiter & I have yet to work out where to fit & what value a capacitor for smoothing the o/p of the receiver. That is the sort of info I was hoping to get by posting here as well as confirmation as to whether the circuit will work or not.

[ghost123uk]

This has become a bit of an enjoyable learning project as well as a practical gadget.

I know that feeling !
The amount of hours I have spent designing, building and de-bugging circuits when in reality I could have just gone out and bought summat ready made

SCR = a 3 legged device very like a tranny in operation, but when it's base ("gate" on an SCR) receives a voltage, it "latches on" and the other 2 legs ("source" and "drain") continue to conduct even when the voltage on the gate has gone. This makes them like a monostable switch, therefore useful to act as a switch when only a pulse trigger is available (as per the o/p frpm your rx) They can be turned off by a brief interruption to the feed or load circuit (source and drain legs)

The 200 ohm resistor is supposed to be the current limiter & I have yet to work out where to fit & what value a capacitor for smoothing the o/p of the receiver.

I don't think you need either, the input to the flip flop is high impedance, so won't overload the o/p of the rx, plus you don't need/want a cap in that bit, it is more likely to do harm than good. Reason = when a pulse arrives it will first have to charge the cap (via a resistor, if used, upstream of course) and this might distort the pulse and cause the flip flop to misbehave. If you did fit a cap, it would only be a very small value (nF rather than uF) due to the high impedance input of the chip.

confirmation as to whether the circuit will work or not.

It's never that simple when designing a one off, it's all down to getting as close as you can the de-bugging.

[ghost123uk]

Just had another thought. When the o/p of the rx is NOT active, I wonder if it is clamped to ground or simply "floating" ?

If it is clamped to ground, even via an internal resistance, then that is good, but if it is floating, the the input to the chip is floating too and that can easily cause instability. You might/will have to put a resistor from the IC input to ground (no cap) to tie it to ground when no signal is present. I tend to use pre-sets and a scope (or analogue meter) to try different values in circumstances like that. Too little resistance and the o/p of the rx is reduced, too much and the IC will be unstable when no pulse is present. I would likely use a 500k pre-set to start with, to do some tests.

[jamesc76]

As much as that looks good you do realize those 12v fan heaters are totally crap, very inefficient and not worth the hassle! just use ya mirrors on the sides to see behind ya!

[ghost123uk]

I think Jeff is just doing a "climb mount Everest, just because it's there" thing and I know the feeling - I have been working on summat totally stupid and pointless for 3 days now, on and off, just because I want the satisfaction of saying to myself "I did it"

[Jeff J]

As much as that looks good you do realize those 12v fan heaters are totally crap, very inefficient and not worth the hassle! just use ya mirrors on the sides to see behind ya!

I actually bought the fan to blow warm air into the rear of the vehicle, which it did & only thought of using it to de-mist the rear screen later, which it also did to an acceptable level, so much as I hate to disagree with anyone, I feel it is in fact worth the effort. In any case having retired, time is something else I am not short of.

[Jeff J]

Thanks Ghost, so can I take it that theoretically the circuit should work, but may be erratic in behavior ? My simplistic thinking was that once the flip-flop chip makes the transistor conduct then it does not need any further input from the receiver & the relay will just be held "on" from the 12v via the transistor. Then once the receiver is pulsed again it will change state thereby switching off the transistor & thus the relay.
If the o/p from the receiver was clamped to ground surely it would never work would it? Would I be able to measure a resistance between the data line & gnd if it was clamped ,with the supply off?

[ghost123uk]

Thanks Ghost, so can I take it that theoretically the circuit should work, but may be erratic in behavior ? My simplistic thinking was that once the flip-flop chip makes the transistor conduct then it does not need any further input from the receiver & the relay will just be held "on" from the 12v via the transistor. Then once the receiver is pulsed again it will change state thereby switching off the transistor & thus the relay.

Yes that sums it up

If the o/p from the receiver was clamped to ground surely it would never work would it?

Clamped to ground would be via a fairly high value resistor (xxK perhaps) so it's output when the pulse appears is still there, just reduced very slightly, BUT, when the pulse is not there, the output is not "floating" but has a resistive path to ground.

Would I be able to measure a resistance between the data line & gnd if it was clamped ,with the supply off?

No you can't do that because in cmos (and most other ic's) this will not be done by simple resistance but is done with complex "active" semiconductor junctions. A close examination of the data sheet will tell you though, but they can be complex to understand. I usually prefer a "suck it and see" approach, and then de-bug any thing that needs it.

[Jeff J]

Thanks for the help, I don't understand most of the gobbledygook on the data sheets, but I now feel more confident about assembling all the components & seeing what happens. I've gotten as far as setting up the voltage regulator & receiver & connecting an led to the o/p of the transmitter which lit up during the time the fob switch was pressed, so I'll fit the other bits now & see what happens.

[ghost123uk]

Remember cmos chips are uber sensitive to damage from static in your body. Wear that wrist earth strap !

That's one of the troubles with cmos, you don't know if you got the circuit wrong, or whether you blew the chip handling it