good morning to everyone, i have just found this site and hope somebody can help as im stuck!!!

what i need:
i require a circuit using a 555 timer to drive a relay but here is the tricky bit,
this will need to be a monostable trigger circuit that uses trimpots to decide

1: the sensitivity of the trigger

2: duration of output from relay

the input source will be from the cars accelerator pedal (potentiometer) which reads between 0v and 5v,
what i need is something that can determine when the pedal is backing off (dropping to 0v) not accelerating (rising to max 5v)

i know this kit exists as i have a top picture of the circuit in question but no values or circuit diagrams - if needed i could post the pic.
these modules are extremely expensive and the company who used to make them no longer provide them.

so to sum up i require a monostable circuit that triggers when the input trigger falls from anywhere between 5v and 0v, the voltage is not important it just needs to "sense" that the voltage is dropping then trigger a relay for a max duration of 3s but must reset when voltage starts to rise again ( so when i push the pedal down by any ammount the system would reset then as soon as i let the pedal come up it would trigger for max 3s then switch the relay off???????????

i know this is one hell of a brain freeze or at least it has been for me.
i have limited knowledge of electronics but can learn extremely quickly.
i have mad various circuits but they will only trigger from a set voltage not a falling one


many thanks in advanve
graham

 

By "pedal" do you mean "gas pedal" or "accelerator"? Or are you just meaning a switch?

Can you post the picture you are referring to?

 

hi there , thanks for taking the time to reply.

yes i mean the gas pedal, most modern cars use drive by wire now , which means that the gas pedal is now basically a petentiometer with on e perticular wire im interested in which provides a low level signal of between 0v and 5v.

the voltage changes depending on the gas pedals position so full throttle (pedal pushed all the way down) you would have 5 v and with the pedal in the rest position (all the way up) the wire shows 0v (ground)

i need a circuit that can "sense" or detect that the voltage is dropping
ie: it cant just pick up 0v or ground it needs to know that the voltage has just dropped from say for example 3.4v - 3.1v (they were just example figures) but the circuit must not trigger when the voltage increases, plus the circuit must trigger when the voltage drops and then must not trigger again until voltage has risen then falls again

sorry for making this a tough first post but like i say i have seen this system made of 3 555 timers and various other components so i know it can be done , i just cant figure the voltage sense part

if i know how to make something that can sense voltage drop i can manage the rest myself

cheers

 

sorry forgot , here is the picture

 

t1 controls the output duration, t2 controls the input trigger sensitivity and the jumper x decide whether the input triggers on voltage rise or fall

many thanks
graham

 

I'm intrigued by your circuit... can you explain what it's supposed to do?

I ask because I'm a keen car tuner. My car uses PWM for the boost control valve.

 

I would also like to know if you will be using a secondary throttle POT or are you trying to tap into the existing POT.

If you are using the existing, a mistake in your circuit, or even component failure could inject 5v to the line, flooring the accelerator.

 

Strongly agree with retched's post - this gets to be a safety issue. Tapping into existing hardware is going to create potential failure modes and probably create warranty and liability issues.

What is the signal to control? Perhaps we can think of some means of getting what you want without having to wire into critical wiring.

 

hi there, this circuit will allow the dump valve to release pressure to the atmosphere then as soon as this is done it will close.

normal dump valves would remain open as long as they have vacum attached to the dump valve. the above circuit is the best way of fitting a dump valve without losing vital performance.

safety is allways a issue however i will be using the cars pot as there are 2 signals that need to be sent to the ecu to creat full thorttle and they basically work against each other.
first sends 0 - 5 v
second sends a resistance ensuring the vehicle either runs correctly or goes into limp mode (idling in this sort of failure).

like i say the kit pictured above is and has been used for 3 years + for this kit and works brilliantly but nobody will send a pic of the underside otherwise i could have figured it out???

i basically need something that works out i have backed off the throttle even just a little.
my old kits all worked via microswitches fitted to the throttle pedal switching the dump valve on only when pedal was fully at rest, however this leads to a very small delay between backing off and d valve opening creating a much weaker hiss. this only works when backing off completely not just a little.

i know to some it probably seems silly but knowing the kit exists with such old school parts its frustrating not knowing how its done???????

 

I understand your urge for the pedal incorporated functionality, but why not a button on the shifter? (I am guessing this is a standard trans.)

But for the pedal way, Im guessing here, but I suspect a comparator and some sort of hysteresis. A microcontroller could do this easily. You could take a samples of the line voltage and the software compare the results. If sample 2 is greater than or equal to sample 1, (You are accelerating or staying at the same throttle level) then replace sample 1 with sample to. LOOP.

If sample 2 is less than sample 1 (you are releasing the accelerator) trigger valve for (x) seconds.

 

hi there and thanks again for your time,
so before replying i googled what these parts were as at first i didnt quite understand (like i say ill learn quick but am by no means a whizz at circuits)

i then looked at the second version of the circuit im after and this circuit incorparates a comparator and hysterisis. i will post a pic of this circuit.

it has 12v in,
ground in,
ground out for the dump valve solenoid,
switched 12v live out for the dump valve solenoid ,
and then 1 signal wire for the ecu in 0v - 5v

this little circuit then controls 1 555 timer circuit which then drives a single 30 amp relay.

i know this sounds cheeky but i really am not sure on how to create the comparator circuit so i am wondering if someone could post a circuit diagram purely for the comparator circuit with a single output to my 555 timer circuit please ???????

preferably where it could perform how you described above where it monitors the input voltage and when it drops below the voltage it just had it will trigger my 555 circuit

many thanks
again
graham

 

http://www.allaboutcircuits.com/vol_6/chpt_6/2.html

The eBook at the top of every page of this site will give you tons of info.

We can fill in the rest.

From the looks of the board, it appears the bulk of it is to provide a regulated, clean power source to the circuit.

The comparator is the key.

 

right then, i think maybe i am getting somewhere,
comparators can be used to give a output state to switch my timer circuit on? depending on when voltage v+ is higher than v- and i understand this can be inverted great as this is what i need

i then think that using the hysterisis side of the comaparator i am able to change the value of the input reference voltage v+ or v-on a constantly changing manner ie, one minute 3.4v then say 4.2 v????

however how do i make this a rolling loop so the when voltage v- is less than v+ or vice versa the timing circuit will trigger.

i have looked at loads of cicruits and too be honest dont quite follow them in hysterisis form.

i now also understand what you meant by a clean input as in giving positive feedback from the output state of the comparator to the input ref voltage i think??? to stop it inadvertantly switching on again great...

this now sounds like the sort of thing i am looking for so i am now wondering if there are any sample circuits with a constantly changing input ref voltage so ie:
v+= ref constantly changin 0 -5v
v-= the above but after it has changed

example vref could be 3v and then the comparator with some sort of cap/resistor combination would be able to remember just for a fraction of a second what that voltage was and compare that to the v-

so in other words the comparator would be comparing what the vref was to what the vref is now???

sorry if this sounds more complicated than it is but until today i never even knew what these were

many thanks again for your time
this is turning into a right lesson