need help with DIY RPM switch circuit
04-07-03, 11:49 AM
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need help with DIY RPM switch circuit
Hey all,
A quick description of this project – I’m swapping a VDI intake onto my S4 GXL, and need a way to open the VDI at ~5000 rpm. People have used commercially available rpm switches to trigger the VDI solenoid, but I found a DIY circuit diagram that seemed to need only minor adaptation, so I built one. I tried the board I built briefly before putting the car in storage for the winter, and it seemed to work fine – save for some relay ‘chatter’ when the rpm hangs around the (adjustable) set point. This is the link to the website describing the rev limiter circuit.
http://cp_www.tripod.com/rotary/pg13.htm
I have the scheme for adding a relay triggered by a transistor slaved to the chip. Basically, all the rev limiting circuitry off pin 8 is deleted, and a transistor fed from pin 5 would switch the ground leg of the relay. What I need is instructions on how to implement some hysteresis in the circuit to eliminate the chatter. Below is a link to the National Semiconductor datasheet and application notes for the LM2907/2917 chip. In the application notes, pg.6 – fig. 6b, they mention hysteresis can be added to the circuit.
http://www.national.com/an/AN/AN-162.pdf
Would someone be able to figure this out for me? I think a differential of a few hundred rpm between on and off would be enough. I would really appreciate any help with this. Could be very useful for VDI, N/A aux. ports, shift lights............
Thanks,
Curtis
A quick description of this project – I’m swapping a VDI intake onto my S4 GXL, and need a way to open the VDI at ~5000 rpm. People have used commercially available rpm switches to trigger the VDI solenoid, but I found a DIY circuit diagram that seemed to need only minor adaptation, so I built one. I tried the board I built briefly before putting the car in storage for the winter, and it seemed to work fine – save for some relay ‘chatter’ when the rpm hangs around the (adjustable) set point. This is the link to the website describing the rev limiter circuit.
http://cp_www.tripod.com/rotary/pg13.htm
I have the scheme for adding a relay triggered by a transistor slaved to the chip. Basically, all the rev limiting circuitry off pin 8 is deleted, and a transistor fed from pin 5 would switch the ground leg of the relay. What I need is instructions on how to implement some hysteresis in the circuit to eliminate the chatter. Below is a link to the National Semiconductor datasheet and application notes for the LM2907/2917 chip. In the application notes, pg.6 – fig. 6b, they mention hysteresis can be added to the circuit.
http://www.national.com/an/AN/AN-162.pdf
Would someone be able to figure this out for me? I think a differential of a few hundred rpm between on and off would be enough. I would really appreciate any help with this. Could be very useful for VDI, N/A aux. ports, shift lights............
Thanks,
Curtis
04-07-03, 02:14 PM
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well remember how I was doing the swap as well, the guy doing it for me removed the ACV and hooked up the 6 ports to the MT headers. Since the ACV was removed the air pump was also removed. w/o the air pump, how is the VDI going to open right?? he hooked it up to a large vacuum source ( I don't know where) but also said that the solenoid to activate it was simply blocking it, so instead under full throttle accel it will open naturally. althouhg I don't have exact RPM control, it opens with enough vacuum source.
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