Fuel Transfer Controller – Lower Cost Version

Figure 1. Schematic Diagram, Digital TDR Fuel Transfer Controller
Figure 1. Schematic Diagram, Digital TDR Fuel Transfer Controller

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.

Figure 2. Mounting suggestion, Digital TDR Fuel Transfer Controller
Figure 2. Mounting suggestion, Digital TDR Fuel Transfer Controller

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.

Figure 3. Board layout, Digital TDR Fuel Transfer Controller
Figure 3. Board layout, Digital TDR Fuel Transfer Controller

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

Qty Mouser Stock No. Description Cost
1 700-ICM7240IPE+ Programmable Timer/Counter – Maxim 5.98
1 653-G5LE-1-DC12 Relay, 12v 34ma, 10A SPDT – Omron 0.88
2 512-2N4401TA NPN Small Signal Transistor 40V 600ma 0.12
25 512-1N914 Small Signal Diode – Fairchild 0.50
8 660-CF1/4C Resistors: 470, 470, 1k, 4700, 10k, 10k, 33k, 6.2M 1.20
1 80-C340C105K5R Capacitor Timing 1µfd 1.06
1 80-C320C103K5R Capacitor bypass 0.01µfd 0.10
    Time Delay Relay Substitution Cost: 9.84
2 696-SSI-LXH1090YD LED Panel Mount Indicators Yellow 10mm Lumex 2.84
2 506-A107SYZQ04 Sw 1, Sw 2 Toggle On-Off-(On) ( ) = Momentary 6.50
1 112-R13-23A-R Sw 3 Square Cap 0.6″ Mount 1/2″ Round 1.40
1 642-FMC12A220 Sw 4 Rocker 10amp Mount 3/4″ x 1/2″ 1.55
1 105-SR2511F-34FN Sw 5 Rotary 3P4T (only 1 pole used) 2.77
1 45KN012-GRX Knob, Pointer 1.25″ x 0.65″ H 0.69
    Total Controller Parts Cost: 25.59


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

Previous articleSpecial Introductory Pricing on Superior-Powered Sportsman: $154,995
Next articleLarge Baggage Door for the GlaStar
Tom Kuffel
Tom Kuffel, builder of Sportsman 7183, is a 4000-hour CFI with sea and multi-engine ratings. He has also flown on several hundred aircraft searches while managing to be the subject of one or two himself.