J&S monitor

my air fuel ratio meter.jpg (27711 bytes)

The design for a J&S monitor is nearly identical to that of an O2 monitor.  The J&S-supplied display is nothing more than an LM3914, LEDs, and a light-sensitive resistor to control the LED  brightness.  The J&S can take out a maximum of 20 degrees retard.  The J&S output is about 0.125V per 2-degree step, so full scale gives a voltage of 1.25V, which not coincidentally corresponds to the natural full scale of the LM3914.  So, with this setup, each LED corresponds to 2 of timing taken out.

As you can see on the picture, I constructed a tiny box and put both the J&S monitor as well as my special O2 monitor in it.

The J&S can be switched to retard all cylinders in response to hearing a knock, or to retard only the cylinder that has knocked (they fire in sequence so it's easy for the J&S to work this out). When the monitor signal is sent out, it is a voltage that represents the amount of retard on each cylinder. If these are all the same because the J&S is set to "retard all", then the same LED will be lit four times in succession (for a four cylinder engine) and then a new value may appear. If the voltages are not the same, different LEDs will be lit, but this happens so fast it *looks* like more than one is lit at the same time.  So the individual cylinder voltages are time-multiplexed to the display which is set to 'dot' mode and is fast enough to light different dots for the different cylinder voltages.  

One resistor (R) is enough to make the monitor.  It controls the LED current, which is 12.5/R milliamps.  The max allowable current for a LED is about 15mA.  So if R=1.25K ohm, there would be 10mA going through the LEDs.  1kOhm will do fine too (12.5mA). 
If you want, you could place an LDR (light dependent resistor or photoresistor) in series with the resistor.  The value of the LDR is selected so that the two together yield about 1K with maximum light, and about 5K-10K in darkness.  Some types of LDRs will require a 4.7K resistor in parallel with them because they go up to 500K-800K in darkness. 
You could add a 100K/0.1F RC filter to the input to suppress ignition noise.

The J&S uses a normal 3.5mm stereo jack on its output which not only delivers the signal for the monitor, but it also acts as voltage source.  Therefore, connect V+ of your monitor to the tip of the jack, earth to the barrel and signal to the ring contact of the jack.  Plug that into the J&S and enjoy your new monitor!