Okay, I just know I’m going to feel stupid when someone explains this to me - but I’m at a loss. I have built a circuit based on a 16F268. It’s powered by a 9 volt battery, run through a 7805 to regulate the voltage to 5 volts. The PIC sends a signal to a cheap 434 Mhtz transmitter. My issue is that the circuit has to be held to 5 volts for the PIC, but I’d like to power the transmitter with 12 volts to increase range. My NOOB question is… How?? If I use a separate battery with 12 volts for the transmitter, including a separate ground, then the signal out from the PIC will not have a ground connection back to the original circuit (9 v.) - intuition says that this won’t work. If I use 12 volts for the transmitter and tie the ground back to the original circuit’s ground, am I going to burn up my PIC? Similarly, if I power the whole circuit with 12 volts, use the regulator to send only 5 volts to the PIC, and run a separate 12 v. connection from the battery to the transmitter, won’t this cause the same burned up PIC, due to the shared ground? I’m sure their is a simple and obvious way of using devices with different voltage requirements, but I’m completely blank and frustrated as to what that is.
Thanks to anyone able to give me a push in the right direction.
You can run different circuits at different voltage and have the “grounds” (I prefer the term “common”) tied together.
The circuits at different voltages if interconnected (PIC to Xmitter) must have a common connection.
To only place where this could be a problem is if the driving circuit (output) has a voltage too big for the receiving (input) circuit. Then there are other circuits that can translate the output from on to the input of the other.
Post a link to the data sheet of the 434 MHz xmitter (hopefully it will have a schematic or at least input specs) so we can take a look to see if there are any ‘gotas’.
Waltr… wow, thanks for the quick and solid reply!! I’m surprised (though happily) with your answer; I (being a NOOB) was concerned that having 2 different voltage inputs and only 1 combined ground or common that somehow the 12 volts would be able to run back along this common line and reach the components that can’t handle that much. I’m wrong alot, but this time I’m happy about it. Just FYI, the link you asked for - the data sheet for the transmitter is: http://www.robotshop.com/content/PDF/tr … 4-2-5n.pdf Obviously I have little experience reading these things, but I’m certain that it says the transmitter can handle 12 volts… I’m just afraid about the components that I know cannot. Another experienced hobbiest offered me the advice that “if you’re not burning up a few components here and there, you’re not really learning electronics.” So I’m going to give it a try.
jremington… I didn’t find it very meaningful either, but I don’t know enough to know whether or not it is very good. However, in a table labled “Electrical Characteristics”, there is a listing for “Power Source” with values for min (3 volts) and max (12 volts). I did as was suggested and connected a 12 volt battery to the transmitter and tied the neg of the battery to the exisiting ground rail – nothing got hot or caught fire and the transmitter did function properly, so I guess that’s a good thing, but it did cause the circuit to act erratically in that a signal going into the PIC from a comparator keeps brining the input pin high immediately upon power up – without the 12 volt battery, the comparator would wait for a microphone to supply a current and then the output from the comparator would go high and take the input pin high, triggering an interrupt on the PIC. Anythoughts on what might be causing this change given that the only difference between the working and non-working circuit is the addition of the 12 volt battery to power the transmitter?
Just a thought… I have ‘pull up’ resistors holding all of the other input pins on the PIC high (with switches that take them low for input) – I know there is such a thing as a ‘pull down’ resistor but I’ve never actually implemented one. I’m wondering if it wouldn’t be a good idea to put a ‘pull down’ resistor on the pin in question so that it is forced low in case the issue I’m having is resulting from ‘noise’ on the floating input pin – would this be a reasonable avenue to troubleshoot?
My Adobe pdf reader complains about a “missing plugin”, so maybe that is where the “Electrical characteristics” table is hidden (for me, anyway).
It is certainly possible to power two connected devices with different power supplies, but it is essential that they share a common ground. Without more information you are correct to worry that 12 V power could feed back into the PIC from the DATA pin and destroy it or cause erratic behavior, but that sometimes can be taken care of by putting a resistor (e.g. 27-47 Kohms) in the lead to limit the current. As waltr pointed out a 5V signal from the PIC pin might not switch the transmitter if it is powered by 12 V, in which case you will need an interface circuit.
To increase signal range use good 434 MHz whip antennas on the transmitter and receiver, and then just run the transmitter on 5V. I’ve been able to get 1000’ range using cheap modules powered at 5V, with whip antennas from http://www.rentron.com
jremington:
Without more information you are correct to worry that 12 V power could feed back into the PIC from the DATA pin and destroy it or cause erratic behavior, but that sometimes can be taken care of by putting a resistor (e.g. 27-47 Kohms) in the lead to limit the current.
That’s it… that’s what I’ve been hoping to find out about, how to protect the PIC from the voltage of the tansmitter coming back through the DATA pin. when you say put a resistor in “the lead” to limit the current, what lead are we talking about? I’m guessing you mean the connection from the PIC’s output pin to the DATA pin on the transmitter? I will definitely put one in there; it can’t hurt anything, right? Better to have it and not need it…
Oh, wait though… won’t a resisitor strong enough to drop 12 volts down to 5 from DATA to PIC, cause the 5 volts coming out of the PIC going to the DATA pin to stop entirely??
Thanks so much, all of you – being new to the hobby and to the board, I can’t tell you how impressed I am with the community here.
People use resistors to limit current to INPUT pins on microprocessors, when those inputs are connected to signal sources that have voltages exceeding the microprocessor power supply. In that case, a limited amount of current flows from the signal source to the uP power supply through the resistor and an internal diode on the uP input pin.
The effect of placing a protective resistor between the PIC output pin and the transmitter data pin depends on the internal circuitry of the transmitter, which is unknown to us. You are correct, the resistor might prevent the microprocessor signal from activating the transmitter. However, if the DATA pin on the transmitter is to the base lead of an internal transistor, then the connection might work (with or without the series resistor).
I suggest to stick with 5V for both devices, or switch to a 12 V transmitter than has better documentation and published interface circuitry that is known to work.
jremington – thanks again. You have a great way of explaining this stuff without making me feel like you’re talking down to me. I really appreciate it. I get that without better information, we can’t “trust” the transmitter, and I get the issue about the resistor, but that lead me to one final thought – what about putting a diode between the PIC output and the DATA pin?? Wouldn’t that let the signal through without letting feedback back into the PIC?
If not, I’ll probably just stick with the 5 volts and look towards gaining range via the attennae – but since the transmitter side of the unit is meant to be portable - worn on a belt like a pager or cell phone - a whip on that end isn’t really feasible. Would adding a whip antenna to only the base/reciever unit add value (range)?
Adding antenna gain on either end of the RF link can increase range. This is how NASA can receive the planetary probes that have low power transmitters. They use huge receiving antennas.
A series diode on the PIC’s output may or may not work. It again depends on the transmitters input circuit.
I would be examining the transmitter and ‘back engineering’ the data input circuit and drawing a schematic of it. Then we could determine a proper and safe interface to the PIC.
Thanks, waltr… you, jremington, and skimask seem to be the “go-to” guys around here – I’m sure I’m not the only one that appreciates all of y’alls willingness to help.
I’ve finished breadboarding the circuit including the diode on the theory that it can’t hurt to have it and powering the transmitter with 9 volts; seems to be working well. I’ve been able to do some very basic testing – taking the breadboards into the backyard – and I seem to get pretty reliable results (esitmated 90+%) at maybe 65 to 70 feet, which is the range at which it is intended to work when completed. I have also ordered a whip antenna (as jremington recommended) to mount on the receiver unit; hopefully, this will improve the reliability even further.
So I think I’m there… at least for the design/breadboard stage. Now for the next terrifying/exciting phase – it’s time to get this stuff off the breadboards and onto more permanent circuit boards. Now, which end of the soldering iron do I actually hold onto again???
