In early May I built a Superregenerative FM receiver, and blogged that I should build a 1.5V to 9V DC-DC converter for it. After testing the second DC-DC converter for the DMM, I think that I can use the same Night Joule Thief board for this receiver. A few of the component values can be changed to reduce the 9V output current from 6 milliamps to 1 or 2 mA, which is enough for the receiver. This will also reduce the current needed from the 1.5V cell, which was already only about 10 mA when the DMM was on and 5 mA with no load. I figure that the no load current can be reduced below 5 mA, possibly 3 mA and with the receiver on, possibly only 6 or 7 mA from the 1.5V cell. With 7 mA load on a AA cell of 2000 mAh capacity, that would give about 285 hours, or about 10 to 12 days running 24 hours a day. With a switch and using it 4 hours a day, that would be about two months.
My one concern is if the converter will cause RF interference in the receiver. I built something similar for my AM-FM radio, and the interference can be heard loudly on the AM band, however the FM band works okay. But this is a full superhet receiver with more noise immunity than a superregenerative receiver. I guess I’ll have to try it and see. I have several more things I can do, such as adding a RFI suppressor sleeve to the 9V wiring from the converter. I could also enclose it in an Altoids tin*, which was what I did for the earlier converter. The metal tin should shield a large part of the RFI.
Update June 13 — I put together a third Night Joule thief board. For R1 I used a 1k and for R2 and R2 I used 22k. The capacitor was 150 pF, same as the other boards. The transistors were PN2222A for both NPNs and 2N4403 for the PNP. The coil was a 100 uH small radial choke. Both filter capacitors were 470 uF. The rectifier was a 1N4148, and the zener was the emitter to base junction of an old transistor.
The output voltage was 9.2 volts, no load. The supply current with no load was well under 5 mA, probably 3 mA but it’s hard to read the very low current on a 150 mA analog meter. With a 4500 ohm load, which is equal to 2 mA, the supply current went up to about 27 mA, and the output voltage was about 9.05 volts. This will be plenty of current for the superregen receiver, which uses only about 1.1 mA.
With a 2000 mAh alkaline cell and assuming the full 27 mA battery current, the converter should run for 74 hours, or more than three days running 24 hours a day. At 4 hours a day, that would be about 18 days. And that’s worst case. With a 1.1 mA current at 9V, the battery current should be less than 20 mA and the batery life should increase accordingly. But the overall cost for 1.5V AA cells is so much lower than 9V batteries that it will soon pay for itself in saving the cost of 9V batteries. For even more savings, use one or two NiMH rechargeable cells.
Now I have to try it out on the receiver, and see of it will not cause interference with the receiver.
*However the Night Joule Thief board is made to have a paper clip loop soldered into each end and the battery is held between these loops. So it is nice and small, and doesn’t need a container as large as an Altoids tin.
Back to experimenting…