I need a 3V ( 2 1.5V batteries) for a RC car transmitter and I have a Raspberry Pi Model B giving out 3.3V. How can I convert the 3.3V from Raspberry Pi to use it in my RC car Transmitter ? Basically I'm trying to run the RC car with the Raspberry Pi GPIO pins 1, 3V3OUT, and Pin 39 GND.
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4\$\begingroup\$ Have you verified that you can't get away with 3.3V? Many parts and circuits do have some leeway in what they'll accept. \$\endgroup\$– Ignacio Vazquez-AbramsCommented Sep 13, 2015 at 10:04
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\$\begingroup\$ Yes, It did not work. I sent 3.3V from RPI to 3V RC car transmitter and it did not work. I checked with multimeter that 3.3 V from RPI was in fact at the RC 2 car Tranmitter pins. So, when I replaced by 2 1.5 V batteries - the RC car Tx worked. \$\endgroup\$– Shashi KiranCommented Sep 15, 2015 at 5:01
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2\$\begingroup\$ You know that you can only draw a few tens of milliamps from the RPi's 3.3V supply, right? \$\endgroup\$– Ignacio Vazquez-AbramsCommented Sep 15, 2015 at 5:01
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1\$\begingroup\$ The problem is not with excessive voltage (two brand new alkalines will give close to 3.3v) but rather probably the opposite - the available voltage will sag under excessive load. \$\endgroup\$– Chris StrattonCommented Oct 14, 2015 at 0:47
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3 Answers
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I think the additional 0.3V won't bite.
Should you really be concerned about this you can get a Schottky rectifier like 1N5819 to drop the voltage by 0.2V.
By the way 1N5819 are so cheap that you can get boxes of thousand for a few bucks - you can get a few hundred of those and start replacing 1N4001's in your circuits with those, and in some circuitry its lower forward drop means better efficiency.
answered Sep 13, 2015 at 18:14
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\$\begingroup\$ Thanks for replies. Is Schottky rectifier same as Schottky diode ? \$\endgroup\$ Commented Sep 15, 2015 at 4:57
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The voltage specifications for the RaspberryPi GPIO don't appear to be available, but this website has done some comparisons to similar parts. Specifically, the section "Input and output pin electrical specifications" shows that comparable parts have a minimum Input High Voltage (VIH) under 2.5V. As long as your part has a minimum output high voltage (VOH) greater than 2.5V, you're probably going to be fine. You didn't provide a part number for your radio transmitter, so make sure you check the spec for that to verify that it can handle 3.3V -- as others have suggested, it shouldn't cause any problems.
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\$\begingroup\$ I tried the 3.3V RPI Output to 3V Transmitter ( that was originally powered by 2 1.5V batteries) and the Transmitter did NOT work. Hence looking at lowering the RPI down by 0.3 V. Thanks for reply. \$\endgroup\$ Commented Sep 15, 2015 at 4:59
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\$\begingroup\$ You are confusing the I/O voltages of the processor, which is not involved, with the capabilities of the on-board regulator which is, hence your posting is irrelevant to the question at hand. \$\endgroup\$ Commented Oct 14, 2015 at 0:48
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The voltage of an alkaline cell can vary from ~1.65V when brand new under no load to ~0.9V when discharged (or even lower if under heavy load).(Alkaline Battery - Wikipedia)
The RC car transmitter should have been designed to operate over this range so powering it directly from the RaspberryPi is a good way to go.
As pointed out by Chris many products do not get the maximum life out of the batteries and may stop functioning before the 0.9V per cell limit is reached.
What is probably more important is that any logic signal you connect to the device is consistent with its power supply - for example powering it from a lower voltage but using the 3.3V GPIO may cause problems.
answered Oct 13, 2015 at 21:13
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\$\begingroup\$ You are correct about the high range of the Alkaline voltage, but wrong about the rest. Most battery powered devices will not operate down to anywhere near 0.9v. Likely the problem, as already suggest by others, is that the supply cannot provide the necessary current. \$\endgroup\$ Commented Oct 14, 2015 at 0:47
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\$\begingroup\$ If you want to get the best battery life the recommended end of life point is 0.9V per cell. If a designer doesn't care about getting the best life, then yes they may not bother to adhere to that. Semiconductors intended to run off of 2 cells are usually specified at 1.8v. \$\endgroup\$ Commented Oct 14, 2015 at 2:10
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\$\begingroup\$ Practically, most 2 cells devices do not work down to 0.9v per cell. It would be great if they did, but it isn't always feasible to make them that way. \$\endgroup\$ Commented Oct 14, 2015 at 3:01