"Only flickers on cranking."

Do you mean your light flickers on cranking, but at the selected RPM is off? On? Why should it flicker on cranking anyway? Or did you set the pots for a very low RPM so you could test it at cranking speed?

I think Zeners have car poltergeists. I built an LCD panel meter to read my O2 sensor (before I realized this was pretty much useless information with the inexpensive sensors anyway) and powered it with 9V. I built a Zener regulator to get the 9V I needed in a package which would fit on the top of a 9V battery connector (so I could also use a battery to run it if using car power introduced fuzziness). It worked great from my bench power supply. In the car it didn't work, even from the car battery with the engine off. So on that anecdotal evidence I curse zeners. [I ended up using 9v batteries, but kept forgetting to turn themoff when not using the car. And in hot weather the front mounted oil tank heated the display to where it went all black - LEDs would have been better. So I haven't installed an ignition powered relay to connect the 9v battery to combat forgetfullness.]

But what if you use an external power source in the car?

I breadboarded a Tach D/A converter and a separate four comparator chip and four pots to see if I could get a speed signal to lock out gears successively as speed increased and avoid missed shifts. Problematic was the low frequency involved even at fairly high speeds (800 wheel revs/mile x 6 CV bolts, you do the frequency math at any speed you might want to pick) and the hysteresis needed to make sure things like solenoids didn't chatter away. I suppose that at engine speeds x 3 the frequencies of interest (redline or close) are less of a problem?

Walt Fricke (with yet another half done project)

Thanks Peter ! sounds good Walt will try that. Warren any idea,s ?





http://forums.pelicanparts.com/uploa...1124833776.jpg

I'm not understanding the voltage regulator shown in the diagram. Where is the regulated output to the LM2907? Is it from pin 3 of the LM 2940 to the other places marked "12 v"? If so, the regulator needs more than 12 volts input to get a regulated 12 volts out. Here is a possible explanation for the flicker: When the starter is cranked, every time there is a compression stroke, the starter sucks in more current, dropping the battery voltage a slight amount. If this dip in voltage goes below the 12 volt level, the regulator won't regulate during this interval. This could upset the LM2907 during the cranking interval.

But I could be wrong...

Tacho circuit replacement
 

Warren,

I have used this chip to repair my tacho. I could not find replacement parts for VDO circuit so I decided to build my own using breadboard. I downloaded the data sheets and also found a useful article at http://www.rgp.nl/bird/BritishWheels.htm
(BTW there was one typo in the circuit dig, where PIN12 should be grounded also).

To check that it worked on the bench, I had to use a frequency generator, and then adjusted the Pot Resistor to match signal revs with dial reading.

Here are pics of how I attached the breadboard to the rear of the dial.

http://forums.pelicanparts.com/uploa...1124891922.jpg

http://forums.pelicanparts.com/uploa...1124891936.jpg

Greetings all, I am very excited at the prospect of potentially finding the problem with my shift light! To answer Walt; I have the revs set to about 6000 RPM on the bench with a frequency generator. It is literally at the start of cranking that it flickers a few times and that is all. While out on track, it does not illuminate ever (my rev limiter is 7300). The circuit is wired into the car power, so Walt's idea of an independant power supply is interesting. As for Jim's question of the regulated output, we don't explicitly show the lines. The regulator output is shown as 12v on pin 3. At the top of the circuit, you will see 3 other points that are the recipients of the regulated power, also marked as a 12v pad. By the way, as a result of Jim's question, we noted that 12 volt is an error. It is actually 10 volt, so as to have some headroom. Hopefully soon we can update a revised drawing (point out any other discrepancies). Keep firing questions cause I think we can have a nice little device for about 5 to 6 dollar total investment! Peter

Peter,

I see one major, and a couple of minor problems ...

The datasheet for the LM2940 regulator specifies that a minimum value of 22 mF must be used on the output terminal ... or instability will occur. You can change the value of the capacitor, or just add a 22 mF tantalum cap to that output terminal.

Your '72 CDI uses nothing but a 33 Ohm pullup resistor as a switched load for the points, so minimal transients [no inductive spikes at all] will be present on the input ... so the two diodes on the input line at pin 1 of the IC are really not needed, or you could put a single SA15A Transzorb (rated at 15 Volts) there. The 12 Volt Zener could actually introduce switching noise into the circuit rather than eliminate it from the signal!

The transient protection and hysteresis adjustment circuitry are probably superfluous because of the Voltage regulator IC, and I suggest eliminating those components until you have the 'basic' speed switch circuit (fig. 9) shown on page 8 of the application note below.

Likewise, I suggest driving the LED or lamp from the output terminal 8, so you can eliminate the PNP transistor and its' bias resistors.

Once you have a working prototype when connected to the ignition system in the car ... some probing with an oscilloscope should be able to tell if any noise or transients are present that need to be taken care of.

http://forums.pelicanparts.com/uploa...1125088012.jpg

Warren, This is GREAT stuff. I have only a hazy understanding of your advice. As such on Monday I'll relay this info for further guidance and testing. Kinda' goes along with Walt's mentioning zener voodoo. I'll have one more opportunity to test on the track next month.