PS: i tried replacing the pot that was included in the PCB of the PIR unit but it didn’t seem to shorten the length of the “ON” duration-

I think you mean "I tried adjusting the pot" . Probably a sensitivity control.
Steve G

 

I think you mean "I tried adjusting the pot" . Probably a sensitivity control.
Steve G

Your correct - it was for adjusting the length of time it stayed active (sending voltage to its output- which i connected to the relay)
It’s shortest setting was 10sec -
which was too long as mentioned above.

 

Like this?
12V - 0.6Vf = 11.4V.
11.4V ÷ 1000Ω = 0.0114A (11.4mA)
11.4V x 0.0114A = 0.12996Watts (0.13W).
A quarter watt resistor would do just fine. Of course, a half watt resistor will be cooler, but you're still only drawing less than 12mA once the cap has saturated. Assuming 47Ω coil and 100Ω in series, that's going to be an initial current of 78mA and 884mW. There, you'd probably want the 0.5W resistor. And if this is going to trigger frequently then go with a 1W resistor. But if it's only triggered once every 10 minutes, the half watt resistor would be fine. It's only going to see current for a moment. Once the cap charges up the relay will drop out. Once the PIR signal is gone the cap will discharge through the 100Ω in series with 47Ω in series with 1000Ω (1147Ω total); the cap should drain at a current of 10mA (113mW).

 

Nice explanation TonyR but I'm pretty sure this is over the head of the TS.
Steve G

 

Intrigued by the idea of using the current through the cap until charged and the relay drops out - I've wired up a rig. The relay I'm using is 12VDC with a 12V regulated PS. The coil has a cold resistance (no current) of 237Ω. Using a 1000µF cap and 100Ω resistor my relay clicks and holds for about half a second. When I increase the resistance to 150Ω the relay doesn't click in. When I decrease the resistance (150 parallel with 100) to 60Ω the relay kicks in and holds for about the same time. Using a 47Ω resistor - I can not discern any difference. Bypassing the 100Ω resistor all together also yields no change in the performance. Again, this is with MY coil of 237Ω.

It may depend on the actual relay and capacitor. I've tried with a 2200µF cap and got one second. But it took longer to recover; meaning it can be re-triggered no sooner than eight seconds. With the 1000µF it was re-triggerable every three seconds. Bottom line, the 100Ω resistor doesn't seem to make any difference for my rig. The real difference is going to be the coil resistance and the capacitance. Given that the TS states his cold coil resistance is 47Ω, coupled with 100Ω resistance may seem to work. I don't have a coil who's resistance is 47 ohms, so - - - . But it works.

 

That sort of what i was trying to do but it needs a custom circuit i’m guessing- beyond my skills-
if anyone can detail this much appreciated

As has been explained, this circuit is very easy to build. I didn't even put mine on a board. I just slathered some solder on the leads of the cap and resistors and just bonded them temporarily for testing. And the circuit works. I built mine in five minutes (had to find a relay). Made modifications and ran tests. All in all, testing out different possibilities I came up with two caps that can give (with my coil) 1/2 or 1 second. I also determined that MINE worked even without the resistor that goes from the cap to the coil. The 1KΩ bleeder resistor is there to drain the cap. Otherwise it can't be retriggered until the cap finally drains off its residual charge.

I'm pretty sure this is over the head of the TS.

I don't know about that. You can see that a lot of things have gone over my head. Worn the hair off too. But I bodged a YouTube type of solution - you know, where the guy solders the leg of the resistor to the leg of the transistor; few more connections and he has a rudimentary circuit. The circuit you drew (I copied) is almost the simplest circuit I've ever built. I've done the relaxation oscillator, that might be a little easier but a bit more hazardous to build and operate. I don't think huronclean will have any problem at all. And if he does - there's nothing he can really damage. It's a chance to learn and grow. Unless he drops the soldering iron in his lap. And I've done that. Yikes!

 

Thanks guys!
appreciate you sharing your skill and time for this Steve and Tony.

Now to find time and parts!

will report back when i get this rolling.

 

As has been explained, this circuit is very easy to build. I didn't even put mine on a board. I just slathered some solder on the leads of the cap and resistors and just bonded them temporarily for testing. And the circuit works. I built mine in five minutes (had to find a relay). Made modifications and ran tests. All in all, testing out different possibilities I came up with two caps that can give (with my coil) 1/2 or 1 second. I also determined that MINE worked even without the resistor that goes from the cap to the coil. The 1KΩ bleeder resistor is there to drain the cap. Otherwise it can't be retriggered until the cap finally drains off its residual charge.
I don't know about that. You can see that a lot of things have gone over my head. Worn the hair off too. But I bodged a YouTube type of solution - you know, where the guy solders the leg of the resistor to the leg of the transistor; few more connections and he has a rudimentary circuit. The circuit you drew (I copied) is almost the simplest circuit I've ever built. I've done the relaxation oscillator, that might be a little easier but a bit more hazardous to build and operate. I don't think huronclean will have any problem at all. And if he does - there's nothing he can really damage. It's a chance to learn and grow. Unless he drops the soldering iron in his lap. And I've done that. Yikes!

hehehe you got me chuckling with the soldering iron in my lap line!

 

there's nothing he can really damage. It's a chance to learn and grow.

Agree totally.

 

huronclean That's no joke! That and dropping and trying to capture an exacto knife with your legs.

Back in the 80's one of the techs (probably happened to a whole lot more guys and/or gals) was doing a "HiPot" test (High Potential to test the insulation resistance to high voltages). He set the probes (live) down on the bench and they slipped off and into his lap. Upper lap. Very close to the ornaments. Now you got ME laughing my arse off. Tahn was his name. Great guy. Funny as heck too.

Best setup I've ever had: An engineer approached and Tahn said to him "Come here. I'll HiPot you." The engineer said "That's OK. I have high internal resistance." I couldn't resist. I blurted out "Yeah - shit don't conduct!"

 

gnite y'all.

 

LMAO

 

Steve G suggested the capacitor back in post 10 or 11, and I verified it. I have suggested it before and it always works. No, it is not the most precise method but certainly reliable and simple. And even better is that the diode is probably not required because with the capacitor and resistor the current change is not that fast.