Fuel Transfer Controller – Lower Cost Version

The fuel transfer controller in this article using an off-the-shelf Time Delay Relay (TDR) yields a simple, effective circuit for this critical function. But the $40 cost of the TDR gives me heartburn. For those willing to do a little more assembly, a digital version of the same relay can be built for less than $10.

Modification of the controller design I published in May 2009 Kitplanes yields a circuit with the exact same capability for a parts cost of less than $26 at today’s prices even with cosmetically nice indicators and switches. The heart of the digital relay is a 7240 programmable timer and its driver circuitry to a 10 amp relay. See the schematic of Figure 1. With the RC timing constant values shown (6.2Mohm & 1μfd), turn-on times of over 25 minutes can be set in increments of about 6 seconds.

Switches 1 & 2 provide the control functions. Either left or right or both auxiliary tanks can be selected. Either or both can be shut off immediately. And a TDR bypass is available from the momentary-on (Override) contact. For the paranoid, some failure modes can be eliminated by feeding the (Override) contacts directly from the main buss through its own circuit breaker/fuse. Unless an additional switch is also added, the breaker/fuse needs to be pilot accessible and pullable to allow override shutoff.

The switchable variable timing is created with a diode array. In practice it is much simpler than it looks. For each Switch 5 output, the desired time is programmed by connecting inexpensive 2 cent diodes to the desired programming inputs, pins 1-8 of the 7240. For space reasons, these diodes need to be mounted vertically. The time counts associated with the connected pins add together to set the “On” time. For example, connecting only pins 4 (8 counts) and 6 (32 counts) sets an 8 + 32 = 40RC time. In this example it would be 40 x 6.2 = 248 seconds.

Please note, I strongly discourage this transfer protocol. The possibility of dumping fuel by over filling the mains just seems too high. (If it is possible for me to make a mistake, I will.) The methodology described in Kitplanes is to set the controller to transfer 2 gallons a side. Then activating a transfer one hour into a flight and every 30 minutes thereafter gives you a cushion of some 2 hours to remember to do a transfer. This also maximizes your option time if a pump fails which can be a major concern if one waits to transfer the aux fuel all at once. To create this simpler circuit, eliminate Switch 5 and the Diode Array. Then connect the wire which went to the common contact of Switch 5 to the appropriate programming pins of the 7240 (pins 1-8) to get the desired “On” time.

The bill of materials gives the Mouser part numbers and December 2012 prices for the controller parts. Don’t be afraid to order small quantities at www.mouser.com. They are accustomed to supplying hobbyists with courtesy. If demand warrants, I’ll make available a kit, including extensive directions, all of these parts and a 1.25” x 3.8” printed circuit board (layout in Figure 3) for $60, postage paid in the US. Price for the fixed time version would be $56. Figure 4 shows a possible mounting of the components in a 3.125 instrument hole. See this post in the GAOA forum for how to make a bezel for this installation.

Parts list, Digital TDR Fuel Transfer Controller

If you have any questions about building this controller, please contact Tom via the forum.