Required help with timed circuit breaker/timer circuit

[Howieau]

Hey,

Im designing a circuit for electronic gear shifting in a car, using gear shift paddles behind the steering wheel. When the paddle is pulled by the driver, it closes a button behind it and this drives a relay to allow power through to the gear shifting solenoid. The circuit diagram can be seen here;

**broken link removed**

The problem is that if the driver holds down the button for too long then it will burn out the solenoid. The solenoid draws about 40-50A instantaneously when the button is pressed(paddle is pulled). I want to implement a timed circuit where the driver can hold down the button for as long as he/she wants but the circuit will open after a set time so the solenoid isnt damaged. Also as soon as the button is released the circuit resets to its original state ready for the next button push (gear change) - for example if I set the circuit to open after 1 second of the button being held, yet the driver only holds it for 0.5 seconds and then releases, then it will reset at 0.5 seconds. Yet if the driver holds it for 2 seconds, it will reset at the preset time of 1 second.

I have been dabbling with 555 ICs and looked at capacitor/transistor setups but havent found any tutorials that meet my needs. I have good amateur experience with electronics but have never had any experienced with timed circuits.

Any help would be much appreciated cheers.

 

[alec_t]

What you need is a pair of retriggerable monostable circuits (loads of examples on the net). You will also need some sort of lock-out to prevent problems if both paddles are pulled at the same time.

 

[cabelyn]

Hi there,
what about to use a PIC microcontroller?
So, you can, for instance, not allow driver to go shift up if the car already is on last shift, and so one...
As we`re talking about just two ports you can use pic10f200. it cost just some cents and can give a lot "intelligence" to your circuit. It`ll be something like "fly-by-wire".

 

[carbonzit]

Hi there,
what about to use a PIC microcontroller?
So, you can, for instance, not allow driver to go shift up if the car already is on last shift, and so one...

I'd say that's total overkill. You could do it that way, if you really just love programming those little critters, but why?

So far as your feature goes, I'm sure the transmission won't care if you energize the upshift solenoid and it's already in the top gear. No harm, no foul.

As one poster's handle here goes, KISS.

 

[awt]

Hi,

Check out this circuit, hope it will be suitable for your need. Just as you requested I have to modified a monostable circuit which I have worked and simulated it to see if it be good for your need. The stable state of the circuit is ‘high’, when the momentary key is held down, it goes low for a preset period determined by the value of C1 and R2 (change these values for your desired period). When the key is pressed and released before the preset period, the output goes back to its stable state (i.e, high).

Modify it to fit into your system.
good luck,

 

[awt]

Hi,

Check out this circuit, hope it will be suitable for your need. Just as you requested I have to modified a monostable circuit which I have worked and simulated it to see if it be good for your need. The stable state of the circuit is ‘high’, when the momentary key is held down, it goes low for a preset period determined by the value of C1 and R2 (change these values for your desired period). When the key is pressed and released before the preset period, the output goes back to its stable state (i.e, high).

**broken link removed**

Modify it to fit into your system.
good luck,

 

[Howieau]

Hey thanks for your help.

Is this MultiSim that you've used to simulate the circuit awt? Can you please send me the file so I can look at it. Ill PM you my email to send it to.

I need to change so that the stable state of the circuit is ‘low’, then when the momentary key is held down, it goes high for a preset period determined by the value of C1 and R2 (change these values for your desired period). When the key is pressed and released before the preset period, the output goes back to its stable state (low).

How should I change the circuit to achieve this?

Thanks