Need to generate a neg pulse

Thread Starter

Jarmster

Joined May 7, 2020
7
Ok I have a question about generating a neg pulse.

I guess if I give some background to the project it will make it easier to understand what I want.

I installed a car starter. Has an aux wire that can generate negative voltage to activate defrost, seats, steering wheel. Anyway the voltage can be pulsed or latched. Unfortunately my defrost uses latched, constant negative. My seats and steering wheel use pulse. Can I generate a negative pulse triggered by the activation of the latched aux. I only need one .5 sec neg voltage pulse. The aux is only active while the auto start is on. As soon as I put the key in and hit the break, the aux goes dead. I’ve read a thread where they talked about using caps and transistors to generate neg pulse at the caps anode using a transistor to trigger. I was just wondering if there was small circuit I could build to generate a pulse.
 

dl324

Joined Mar 30, 2015
10,764
Welcome to AAC!
Can I generate a negative pulse triggered by the activation of the latched aux. I only need one .5 sec neg voltage pulse.
By negative pulse, I assume you mean a HIGH to LOW transition; and the answer is yes.

It sounds like the aux active would generate a rising edge that could be used to trigger a one shot to give you your half second LOW pulse.

I'll draw a circuit using a 555 timer for you shortly.

EDIT: Add schematic for half second one shot. You still need supply decoupling and be mindful that automotive electrical systems have spikes in the area of 80V.
clipimage.jpg

Using a CD4538 would reduce component count because it can be triggered by either edge and has complementary outputs. May still need to buffer the output because they can't source or sink much current.

EDIT: Circuit using CD4538.
clipimage.jpg
 
Last edited:

MisterBill2

Joined Jan 23, 2018
6,111
Ok I have a question about generating a neg pulse.

I guess if I give some background to the project it will make it easier to understand what I want.

I installed a car starter. Has an aux wire that can generate negative voltage to activate defrost, seats, steering wheel. Anyway the voltage can be pulsed or latched. Unfortunately my defrost uses latched, constant negative. My seats and steering wheel use pulse. Can I generate a negative pulse triggered by the activation of the latched aux. I only need one .5 sec neg voltage pulse. The aux is only active while the auto start is on. As soon as I put the key in and hit the break, the aux goes dead. I’ve read a thread where they talked about using caps and transistors to generate neg pulse at the caps anode using a transistor to trigger. I was just wondering if there was small circuit I could build to generate a pulse.
Now I see that words are left out. It is a remote control starting system, not a starter motor. That makes more sense.
I doubt that it is really a negative pulse. And what sort of current vehicle uses negative voltages?
There is something missing in the description of the system.
 

Thread Starter

Jarmster

Joined May 7, 2020
7
Yes I’m sorry. It’s a remote starter. A fortin evo one and I’m just describing what the documentation is saying as far as the signal used by the aux.
 

MisterBill2

Joined Jan 23, 2018
6,111
OK, now it is much clearer. Product documentation is often not as accurately descriptive as it ought to be, especially after multiple translations done by non-technical individuals. I own a transceiver like that. And so it may be a voltage that rises at some early time in the cycle and then drops back to zero. Or it may be something else. One way to evaluate it is to use a small 12 volt light to see what the signal actually does during a startup sequence.
So what we need to know is how much current that output can provide when it is on. There are simple ways and complicated ways to generate pulses, and some ways to have the pulse isolated, in case the thing being switched does not use a ground-referenced control signal.
 

Thread Starter

Jarmster

Joined May 7, 2020
7
The aux goes to ground. I did hook up a test light. Hooked the lead to a positive and probed the aux connector. When I activate the aux, the test light came on for a .5 sec pulse. How much current I need is the question. It’s a short pulse and it’s only to activate the relays controlling the heated seats and steering wheel. I really have no way to determine that.
 

MisterBill2

Joined Jan 23, 2018
6,111
The aux goes to ground. I did hook up a test light. Hooked the lead to a positive and probed the aux connector. When I activate the aux, the test light came on for a .5 sec pulse. How much current I need is the question. It’s a short pulse and it’s only to activate the relays controlling the heated seats and steering wheel. I really have no way to determine that.
OK, if it is a short pull-down to ground then the really safe approach will be to have it drive a small 2-pole normally open 12 volt coil relay. THAT will provide you with two isolated sets of contacts that you can use in parallel with the switches, with the big benefit of total isolation and no possible interaction. Cheap insurance against interesting troubleshooting work.
 

Thread Starter

Jarmster

Joined May 7, 2020
7
OK, if it is a short pull-down to ground then the really safe approach will be to have it drive a small 2-pole normally open 12 volt coil relay. THAT will provide you with two isolated sets of contacts that you can use in parallel with the switches, with the big benefit of total isolation and no possible interaction. Cheap insurance against interesting troubleshooting work.
That was when the unit was set to pulse. It’s now a latch because the rear defrost uses a latched signal so I changed it. The seats and steering wheel use a pulsed signal. I tried to run the steering wheel heater with the latched signal and it shut off after a few secs. I need to be able to produce the neg pulse using the neg latched signal as a trigger.
The unit only allows for one or the other. I was just looking for a circuit that could produce the pulse when the aux went low. I only need the one pulse to activate the switch relays.
It looks like the circuit provided will perform what I need unless you see something wrong with the circuit. I was thinking about the first one. I have a prototype kit that will produce single sided and double sided boards. I using eagle to generate the board image. Lol it’s been awhile since I’ve used it. I was hoping I could keep it to a single sided but there looks like a couple traces might prevent that.
 
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Thread Starter

Jarmster

Joined May 7, 2020
7
The one using the 555 timer. You said the other might not produce enough current and I figured decoupling with a cap was easier then buffering the output. I dont know. I’m not an expert lol
 

dl324

Joined Mar 30, 2015
10,764
The one using the 555 timer. You said the other might not produce enough current and I figured decoupling with a cap was easier then buffering the output.
You can replace Q2 with a power transistor (TIP29 can handle 1A). Replace R4 with a 100 ohm resistor. That will draw about 100mA from the timer, but it can handle 200mA.

Another option is to replace Q2 with an N channel MOSFET with a sufficiently low \(V_{GS_{(th)}} \).
 

MisterBill2

Joined Jan 23, 2018
6,111
That was when the unit was set to pulse. It’s now a latch because the rear defrost uses a latched signal so I changed it. The seats and steering wheel use a pulsed signal. I tried to run the steering wheel heater with the latched signal and it shut off after a few secs. I need to be able to produce the neg pulse using the neg latched signal as a trigger.
The unit only allows for one or the other. I was just looking for a circuit that could produce the pulse when the aux went low. I only need the one pulse to activate the switch relays.
It looks like the circuit provided will perform what I need unless you see something wrong with the circuit. I was thinking about the first one. I have a prototype kit that will produce single sided and double sided boards. I using eagle to generate the board image. Lol it’s been awhile since I’ve used it. I was hoping I could keep it to a single sided but there looks like a couple traces might prevent that.
One more cheating cheap trick that worked for another person whose system just required a short pulse. In series with that pull-down, put a capacitor, a 0.47 mFD rated at least at 20 volts operating, a bit more would be better. How it works is that as the switch closes from your start-box, the capacitor draws current to charge, pulling the control input down like a solid contact. BUT then, after the capacitor is charged, the current stops flowing, similar to a switch opening. Thatis about as simple as it can get. You will probably need a switch for each system that you are starting with a pulse. That should avoid interactions.
 

Thread Starter

Jarmster

Joined May 7, 2020
7
I think I’m following what your saying. since it’s a neg pulse do I not need the neg side of the cap on the switch side and if I understand you correctly as the cap charges, it creates the neg pulse on the switch side until it’s charged. I’m just trying to understand. Isn’t the line still neg charged even after the current flow stops?
 

MisterBill2

Joined Jan 23, 2018
6,111
I think I’m following what your saying. since it’s a neg pulse do I not need the neg side of the cap on the switch side and if I understand you correctly as the cap charges, it creates the neg pulse on the switch side until it’s charged. I’m just trying to understand. Isn’t the line still neg charged even after the current flow stops?
Actually, the capacitor produces the voltage pulse on the other side until it is done charging. And even better, you could probably use a single capacitor to operate a small multi-pole relay and have isolated contact closures to start each device. The principle is the same as what causes the current inrush when a capacitor filter first has power applied.
 
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