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...


RC Integrator - Unity-Gain OpAmp Buffer - Comparator


How would this work? Any help is greatly appreciated.

On 31 Mar 2005 19:38:05 -0800, "Thomas" put
finger to keyboard and composed:

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...


RC Integrator - Unity-Gain OpAmp Buffer - Comparator

I think you would also need a sample-and-hold.

How would this work? Any help is greatly appreciated.

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:

http://members.optusnet.com.au/~fzabkar/PRNDL2.gif (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
--
Please remove one 's' from my address when replying by email.

On Sat, 02 Apr 2005 13:19:42 +1000, Franc Zabkar
put finger to keyboard and composed:

On 31 Mar 2005 19:38:05 -0800, "Thomas" put
finger to keyboard and composed:

I am trying to activate a relay when the vehicle is in anything other
than Park.

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:

+5V
___
|
_|_
/_\ 1N4148
|
|
P ---|-- 74123
|
_|_
/_\ 1N4148
|
_|_

Ground


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.