Update onthe great clock replacement project.
Several posts above I described revising the gauges on the left most cluster gauge with an oil pressure gauge and a voltage gauge. This replaces the clock in the right-most position of the cluster.
After mucking around with the temperature gauge, which seems to obey some alien laws of electronics, I located an antique Porsche gauge also made by VDO. This has a native oil pressure gauge that uses the same 10-180 ohm VDO oil pressure sender that many European cars use.
These combo meters are "handed", so the meter on the right side of the gauge runs counter-clockwise, with high being up and low being down, and the meter on the left side running clockwise, still with high being up and low being down.
The original gas gauge was a left side (CW) meter, and the temp gauge was right side (CCW) meter.
The Porsche oil pressure gauge was the CCW (right side) type, so I was then able to re-purpose the gas gauge to display volts..
Easier said than done...
I am still not 100 clear on how these meters work, but they appear to have three wires coming from the movement. In the original application (fuel level) the + side of the meter was connected to the battery voltage, one end was connected to ground, and one intermediate wire was connected to the variable resistor in the fuel level sender. So rather than sensing voltage, they are set up to sense resistance.
Converting this to read volts from the battery was something of a puzzle. I could get it to respond by connecting the + terminal to the battery voltage source (variable so I could tests the meter), and connecting ground to the G terminal. Connecting anything to actual case ground had no effect, but you had to have one of the tiny meter movement wires connected to case ground, or the meter would not work.. Don't ask me why...
The core issue I faced was that the meter was actually very linear. I could put a resistor in the battery line that would just peg the meter at 16 volts, but then the scale was from there to 0 volts at the other end. In a car any battery below about 8-9 volts is effectively useless, so having a volt meter that told me that the battery was between 8 and zero volts was pretty useless, and then the difference in pointer position from 12 to 14 volts (the range we really care about) was minuscule.
So, how to create a nonlinear drive for the meter movement? Normally with a solid power supply, this would be easy, just make a circuit that bends the voltages in the way we want.. But in this application the basic idea is that the battery voltage in what is being measured, so we can't design a circuit that depends on a known and fixed battery voltage to function, because that will not be there when we need it..
After dinking around with various diode circuits to try to either steer the current through different resistors to change the range , or to create a nonlinear circuit (both of which ended up requiring negative resistor values), I realized I could simply shift the scale of the meter to the relevant range using as zener diode.
A zener diode acts like a regular diode, except in reverse bias it breaks down and conducts at a specific voltage. So, when the applied voltage is above the zener voltage the diode conducts to set the voltage drop at whatever the zener voltage is (this is a selectable value depending on the diode chosen). Below that value, the diode acts like regular reverse biased diode, that is, it doesn't conduct any current, and the meter goes to zero.
If what we really care about is voltages above, say, 8 volts, then we put an 8 volt zener diode between the meter movement and ground. This means that as long as the voltage applied to the meter is above 8 volts, then the zener will conduct, and a rising applied voltage will cause the meter to move higher, but when the applied voltage drops below the zener voltage the diode does not conduct and the meter will go to zero.
The circuit looks like this:
And the behavior looks like this:
So, I need to redo my gauge labels to support this new scale. Here is what I am thinking: