DUAL INSTRUMENTATION By George Copland Route 1, Box 345 Duncan, Oklahoma 73533 L.S MORE TWO place homebuilts are constructed, there is a need of dual instrumentation for fuel level, oil temperature, oil pressure, in addition to other instruments. Some instruments, such as air speed, oil pressure, etc., need only a Tee in the pressure line for adding a second gauge. Fuel level and oil temperature are not quite that simple. Electrical gauge systems may be used for these functions and are more flexible to use, however, electrical gauges generally require a sender for each gauge. A dual fuel level installation would require two level senders to be installed in the tank which presents a problem. Oil temperature would require a second opening on the engine to mount the sender. I ran into this problem in building my "Starduster Too". As it turns out, the additional sender is more expensive than the solution to be described. Figure 1 shows a single gauge wiring diagram. This is typical of the Ste wart-Warner system. A transistor circuit was designed to connect on the sender terminal of the first gauge to drive a second gauge. Figure 2 shows the circuit comprising two transistors and one resistor. The circuit can be made for less than $2.50. Figure 3 shows how the circuit and second gauge are connected to the first system. The addition of the driver circuit to the first system causes no change in reading of the first gauge. There are slight differences in manufacture and calibration of gauges. One can make the two gauges track each other by the following procedure. First hook up a gauge as in Figure 1. Use a 100 Ohm resistor in place of the sender so a constant temperature bath is not required. A fuel level sender could be used by setting it at

mid-scale and taping it in place so it doesn't move. Note the reading on the gauge. Now exchange the first gauge with the second and note its indication. The gauge with the highest reading will be designated Gauge 2. With reference to Figure 3, a series resistor may be added between the SEN 2 terminal and the SEN terminal on Gauge 2 to reduce the reading of Gauge 2. On the gauge I had, a 10 Ohm resistor gave a 1 needle width reduction, which was sufficient correction. It would be almost as simple to match a couple of gauges from stock if the supplier had several on hand. Figure 4 shows the layout of parts on a 9/ie" x 13 Il /ie" piece of Vectorboard. The two Motorola transistors MJE520 and MJE370 cost 94c and $1.07 from Newark Electronics, 500 N. Pulaki Rd., Chicago, Illinois 60624. The resistor is a 1,000 Ohm, Vz Watt, carbon type, 10% tolerance and costs about lOc. Looking down on top of the plastic transistors with the leads pointing toward you, the emitter is on the left, collector in the center and the base on the right, abbreviated ECB on the circuit diagram Figure 2. A piece of W shrinkable plastic tubing 2" long was slipped over the circuit assembly and shrunk with a heat gun. Figure 5 shows the circuit assembly installed on the rear of a temperature gauge. The plastic loop is already on the gauge. Other gauges would require a different configuration. The gauges I used are the new Stewart-Warner Stage 1, 2, 3 design. These gauges are black styled gauges and really look nice on an instrument panel. The needle movements have been dampened and really work great. I used the Stage 1 for oil temperature (281-C) and a Stage 3 for fuel level (283-M). The oil temperature sender I used was a D-362-CZ which has a Va NPT for mounting. I took an AN % plug and drilled it with an R Drill and taped Va NPT for the sender. This mounts in the rear of an oil screen cover on a Lycoming engine. I feel this circuit will work with the older StewartWarner gauge systems such as the 301-H and D-391-B fuel gauge set. It would be interesting to learn of other applications of the circuit. Q

I2.V + I2.V

_L —— FIG. 2

FIG. 1

42 JULY 1973

FIG. 4

D

SENDER

FIG. 3

FIG. 5