LDM

This video shows the teardown and the test of a fuel flow indicator P/N 65691-005-1 manufactured by the French company Jaeger in the 80s. The last part shows the schematic diagram with explanations.

00:00 - Intro
00:40 - Teardown
02:45 - Power Supply board
03:15 - Servo Amplifier board
03:31 - Fuel flow rate and logic board
05:58 - Test
08:45 - Reverse engineering - Part 1: analog display
10:25 - Digital input circuit
11:44 - Gate and averaging circuits
13:11 - FF rate and display update rate
15:51 - Counters reset signal
16:24 - Scrolling detection
17:40 - Counters and display
19:34 - Clock generator

Connector pinout:

A-B: Lighting, 5Vrms
D-K: 115Vac 400Hz
J: input common
H: analog input, 0 to 5V
C: digital input, 2.27Hz square wave for 1,000. Display proportional to input period. Hi level 6V min.


How it works:
There are 2 two separate inputs. The first one is a DC voltage from 0 to 5V which permits to set the needle between 0 and 12,000 kg/h. Its uses a torque motor with an embedded potentiometer for the feedback. A servo amplifier permits to set the needle according to the input voltage.
The second input is a square or rectangular signal. The clock signal is fed to three CD4518 counters gated by the input signal. An averaging of 10 is performed by counting 10 input cycles and by dividing the counts by 10. A the end of the counting a load signal permits to update the coding wheels display.
The display rate depends on the rate of change of the fuel flow. A differentiator is used to get an image of the variation of the fuel flow with time. This signal is fed to a Voltage Controlled Oscillator which gives the maximum update rate of the display.
The rightmost digit scrolls when there is a variation of the fuel flow.

Schematic diagram:
https://mwblog.fr/wp-content/uploads/...