2005 Ford Transmission PRNDL Logic... need help please.
Apr 01, 2005 2 Replies
T
Thomas
I am trying to activate a relay when the vehicle is in anything other than Park. The transmission module sends out 5V @ 120hz with varying pulse widths. In Park, I read 0.71V with the meter and looks to be apprx 1/7th of the pulse on the scope (square wave). In Reverse, the pulse is measured at 1.6V of 1/3 of the pulse with higher voltages and pulse widths in N,D,2. I've tried using this to enable a small dc relay, but only get chatter (even in Park). I've been told I could do the following...
How would this work? Any help is greatly appreciated.
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F
Franc Zabkar
On 31 Mar 2005 19:38:05 -0800, "Thomas" put finger to keyboard and composed:
I think you would also need a sample-and-hold.
I'd prefer a one-chip solution involving a PIC micro, eg a $3 PICAXE with on-chip BASIC. An intelligent chip could open up additional possibilities.
Otherwise I'd opt for a discrete digital solution involving a 74LS123 retriggerable monostable multivibrator and a 74LS74 flip-flop.
Here's my hand drawn attempt:
formatting link
(7KB)
I've assumed a period of 8.3 msec (=120Hz), and pulse widths of 1.2ms (1/7 x 8.3) and 2.8ms (1/3 x 8.3) for Park and Reverse signalling. I've also assumed the pulses to be positive going.
The 74123 is triggered by the transmission pulse (P) and produces a negative reference pulse, P1 (2.0ms). P1 clocks the 7474 flip-flop on its rising edge, ie after 2.0ms. If pulse P is shorter than P1, then the input to the FF will be low, otherwise it will be high. The Q or Q* outputs can then be used to control a relay via a transistor.
Cext and Rext are chosen so that ...
2.0ms = 0.45 x Cext x Rext. Rext >= 5Kohm.
I've selected a pulse width of 2.0ms because it falls half way between Park and Reverse. Values of Rext = 10K and Cext = 0.47uF would give an acceptable pulse width of 2.1ms.
- Franc Zabkar
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Franc Zabkar
On Sat, 02 Apr 2005 13:19:42 +1000, Franc Zabkar put finger to keyboard and composed:
Oops, the relay driver transistor in my circuit should connect to the Q output of the 7474, not its Q* output. I would also add clamping diodes to the P input of the 74123: