Maker Pro
Maker Pro

Help with time delay relay circuit

steamngn

Aug 27, 2012
22
Joined
Aug 27, 2012
Messages
22
I'm Baaaack...:mad:
Well, I finally got time to get this fuel cutoff installed, and I'm having trouble. The LED portion works ok, proper color depending on the two switch states. However, the buzzer never sounds and the relay doesn't work. When I put a meter on the relay coil wires, it always show s 5 volts. When I connect the buzzer, the relay circuit is showing 12 volts, but I guess this is feedback as the relay never switches.
Any ideas where I should start to look? I went back over my connections, but I'm not sure what to look for....:confused:
Andy
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
8,393
Joined
Nov 28, 2011
Messages
8,393
Hi Andy :)

OK, I'm assuming you're using the design from post #20 on this thread?

So the wires to the relay coil both have 5V on them? There's something wrong there. One of them is connected directly to the positive power rail, which should be 12V. You'd better check that first.

Also, connecting the buzzer shouldn't affect the voltages on the relay coil. It certainly shouldn't cause them to increase!

A few things you can check. You need to use a digital multimeter.

The voltage on C1 should be around 11V while the bottom float switch is open (i.e. fuel level is above the low float). When the bottom float switch closes, the voltage on C1 should drop gradually to zero. Also, while the bottom float switch is open, the voltage on the emitter of Q3 should be less than 0.2V.

Have a look at the power/ground and relay coil wiring and try to find out why there's only 5V on the relay coil wires first. Once you've fixed that, maybe everything will suddenly work. If not, measure the voltages on C1 and the collectors of all the transistors with the fuel level above both floats, and with the fuel level below after waiting a minute or so.
 

steamngn

Aug 27, 2012
22
Joined
Aug 27, 2012
Messages
22
Hey Kris,
I will get on to this first thing Thursday morning... One thing I guess I missed was that the relay is energized when the floats are open, and de-energized on low fuel. In my head I had it the other way around, but either way is good. I am using a digital meter, and will check this out and let you know!
Thanks again for all your help and guidance!
Andy
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
8,393
Joined
Nov 28, 2011
Messages
8,393
One thing I guess I missed was that the relay is energized when the floats are open, and de-energized on low fuel.

You mean the main relay at the output of the circuit? You want it to be energised to enable the engine to run, and de-energised when the fuel runs low, so it will kill the engine? Or are you talking about the signals from the floats themselves?
 

steamngn

Aug 27, 2012
22
Joined
Aug 27, 2012
Messages
22
Yup,
The main relay. My initial thinking was to have the relay NOT energized, and the normally closed contacts wired to allow the fuel hold coil circuit to function. Then, if fuel went low, the main relay would energize and break the circuit to the hold coil shutting down the engine. Either way is fine, I was thinking that not having the main relay normally energized would give it longer service life...
Andy
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
8,393
Joined
Nov 28, 2011
Messages
8,393
Just as a matter of principle I would normally have the relay energised to allow the engine to run. You seemed to be pretty clear that you wanted it the other way, which shouldn't be a problem either, and does save a bit of power while it's running I guess.

Do you want to change the circuit so the output relay is normally energised? Are you using the 555 design or the two-transistor circuit? I've lost track. The 555 design can easily be changed to do this; the transistor one will need at least one more transistor.
 

steamngn

Aug 27, 2012
22
Joined
Aug 27, 2012
Messages
22
Just as a matter of principle I would normally have the relay energised to allow the engine to run. You seemed to be pretty clear that you wanted it the other way, which shouldn't be a problem either, and does save a bit of power while it's running I guess.
It doesn't matter which way it is designed, as long as my dumb ass understands WHICH way! :D
Do you want to change the circuit so the output relay is normally energised? Are you using the 555 design or the two-transistor circuit? I've lost track. The 555 design can easily be changed to do this; the transistor one will need at least one more transistor
I am using the transistor circuit in post #20, and it can stay as-is. Buuut... now I am a little bit confused. In the schematic in post #20, is the relay coil:
  1. Always energized, then de-energizes when fuel runs low
  2. Not energized at all until fuel runs low?
Either way is fine, but I'm not sure what I'm testing!:D
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Joined
May 8, 2012
Messages
4,960
Are you using the 555 design or the two-transistor circuit? I've lost track.

Yes, have you ever considered that what we do here would put the average instructor in thermal meltdown? They usually have all their students studying the same thing at any given time. We multitask!;)

Chris
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
8,393
Joined
Nov 28, 2011
Messages
8,393
I am using the transistor circuit in post #20, and it can stay as-is. Buuut... now I am a little bit confused. In the schematic in post #20, is the relay coil:
  1. Always energized, then de-energizes when fuel runs low
  2. Not energized at all until fuel runs low?
#2. The relay is not normally energised. When the fuel runs low and stays low for long enough, the output relay is energised and stays energised until the fuel rises above the low float or the power is disconnected.
 

steamngn

Aug 27, 2012
22
Joined
Aug 27, 2012
Messages
22
Got it!
Ok, I will have some time first thing Thursday morning to do the checks you provided, and to double check my work... Keep you posted as to what I find!
 

steamngn

Aug 27, 2012
22
Joined
Aug 27, 2012
Messages
22
Ok Kris,
So I said Thursday... Just not WHICH Thursday! :D
So I started going over my circuit, trying to understand what was wrong... Guess what? The two MPSA14 transistors were installed.... SDRAWKCAB! :eek:
Yup, I had the collector and emitter leads reversed on both of them... flipped them around, and TA-DA! Works like a champ!
I will install it on Sunday, and let you know how it works out.
Andy
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Joined
May 8, 2012
Messages
4,960
... Guess what? The two MPSA14 transistors were installed.... SDRAWKCAB! :eek:
Yup, I had the collector and emitter leads reversed on both of them... flipped them around, and TA-DA! Works like a champ!
Andy

I'm sure your broke an unblemished EP record. No nube has ever done that before:... ever! .. Really! :rolleyes:

Chris
 
Top