Need help on a simple (so i thought) circuit - Page 1
Forum section: electronics / beginners

author: nightfox7
author_rank: Contributor
timestamp: November 22, 2014, 08:38:38 pm
content: So, i'm working on a project that will allow me to track the location of a weather balloon, by using a network set up by the HAM radio community called APRS (Automatic Packet Reporting System). On the device that has a GPS connected to it, and that takes the serial data in from the gps, and packetizes it for the APRS network, that then will then be sent out via a radio transmitter, for receivers on the ground to receive and decode the packets, thus giving the location information about where the transmitter is. This isn't really necessary to the understanding of my problem, just wanted to give a bit of background.So, what i'm having problems with is this:On the module that has the GPS connected to it, there is a pin that acts as a push to talk (PTT) to tell the transmitter when to start transmitting. This PTT pin is not grounded when there does not need to be any transmission, then, when the module needs the radio to transmit, that pin is grounded, and the radio starts transmitting. This works perfectly for handheld radios, which require their PTT pin to be grounded to transmit, but I am using a small radio module that is not a handheld. The radio module i am using is the HX1 (Linkhttp://www.radiometrix.com/content/hx1), and it requires 5v on its enable pin, which tells the radio to transmit.So, what i've been attempting to do for the last day and a half is create a simple circuit out of some transistors that will give 5v to the enable pin on the HX1 when the PTT pin on the module with the GPS connected to it. Maybe I'm going the wrong way trying to use transistors? I just had some laying around so i figured I would make it out of that..i've tried many different configurations of transistors that I thought would work, but i still have not succeeded.The picture below is one of my designs that I thought should work.. The switch on ground would be the internal switch in the module that the gps is attached to, and the LED would be the enable pin on the HX1 radio module.

author: tggzzz
author_rank: Super Contributor
timestamp: November 22, 2014, 08:42:35 pm
content: It would help if you would show a diagram indicating whether the transistors are npn or pnp, and which is the emitter and which is the collector.

author: Paul Price
author_rank: Super Contributor
timestamp: November 22, 2014, 08:45:50 pm
content: Your simple circuit is a short circuit.

author: nightfox7
author_rank: Contributor
timestamp: November 22, 2014, 08:51:17 pm
content: Quote from: tggzzz on November 22, 2014, 08:42:35 pmThey are NPN transistors, and in the picture I posted, the collector is on top, emitter is on the bottom. Sorry I neglected to include that info.

author: Rerouter
author_rank: Super Contributor
timestamp: November 22, 2014, 08:57:56 pm
content: I have drawn out how your circuit should be wired to work, and drawn up the circuit you likely desire instead, see how these suit you, as a transistor without any form of current limiting resistor will happily pass 10's to hundreds of amps through its base for the brief moment before it pops,

author: IanB
author_rank: Super Contributor
timestamp: November 22, 2014, 09:15:29 pm
content: OK, Rerouter beat me to it. But when trying to design a circuit, it sometimes helps to sketch out the requirements before putting transistors in it. Given that, I think the circuit you need is like this?If so, you should be able to determine that Rerouter's circuits will fit in the black box and do the right thing.

author: nightfox7
author_rank: Contributor
timestamp: November 22, 2014, 09:18:46 pm
content: Quote from: Rerouter on November 22, 2014, 08:57:56 pmOkay, I'll have a look at that and try to figure out exactly what everything is doing. ThanksI was just drawing this up (in an advanced CAD software) to show what I'm trying to do.The pin 3 on the db9, and the enable on the HX1 is what needs to talk (hence why I left it open).

author: IanB
author_rank: Super Contributor
timestamp: November 22, 2014, 09:19:13 pm
content: Except that Rerouter's first circuit on the left has a floating output when disabled, rather than a 0 V output. This may or may not be a problem, but a pull-down resistor would fix it.

author: nightfox7
author_rank: Contributor
timestamp: November 22, 2014, 09:25:41 pm
content: Forgive my ignorance.. but what are the circles on the emitter legs in rerouter's circuit?

author: IanB
author_rank: Super Contributor
timestamp: November 22, 2014, 09:29:12 pm
content: They represent the "load", i.e. the 5 V enable pin on the HX1. I would personally have drawn the circuit the way I showed it, since we are generating an output level signal rather than powering something.

author: nightfox7
author_rank: Contributor
timestamp: November 22, 2014, 09:43:08 pm
content: Quote from: IanB on November 22, 2014, 09:29:12 pmAwesome, makes enough sense.Do the values of the resistors matter much? at the moment I only have some 220 and 4.7k laying around

author: IanB
author_rank: Super Contributor
timestamp: November 22, 2014, 09:56:23 pm
content: Quote from: Rerouter on November 22, 2014, 08:57:56 pmI'm not fully convinced by these circuits. The one on the left has an open collector output which may not provide a suitable "off" signal when in the resting state.Also all transistors should be off and the current consumption should be zero when the circuit is not enabled to preserve battery power. The circuit on the right does not seem to satisfy this.

author: nightfox7
author_rank: Contributor
timestamp: November 22, 2014, 10:09:40 pm
content: Quote from: IanB on November 22, 2014, 09:56:23 pmWell i'm not sure that the one on the left satisfies my needs, as all i am working with is a ground that is switching between floating and grounded.. but as of right now ive been working on it since this morning so i'm just utterly confused..

author: edavid
author_rank: Super Contributor
timestamp: November 22, 2014, 10:18:58 pm
content: Wow, what a bad datasheet. It says nothing about the drive requirements for the enable pin, so everything here is just guesswork.My guess would be rerouter's first circuit (I would leave out the emitter resistor), but really, ask the vendor for more information.

author: Rory
author_rank: Frequent Contributor
timestamp: November 22, 2014, 10:33:08 pm
content: My suggestion is that you use a PNP BJT for your switch.http://www.rason.org/Projects/transwit/transwit.htm

author: IanB
author_rank: Super Contributor
timestamp: November 22, 2014, 10:36:01 pm
content: Here's a variation on Rerouter's first circuit, with DC analysis in resting and enabled states.

author: nightfox7
author_rank: Contributor
timestamp: November 22, 2014, 10:37:16 pm
content: Quote from: Rory on November 22, 2014, 10:33:08 pmIt looks like that is what i'm looking for.. is there any way to do this with NPN?

author: nightfox7
author_rank: Contributor
timestamp: November 22, 2014, 10:40:20 pm
content: Quote from: IanB on November 22, 2014, 10:36:01 pmOkay i think i can see why that does what it does.That is using a PNP correct?

author: Rory
author_rank: Frequent Contributor
timestamp: November 22, 2014, 10:50:44 pm
content: Quote from: nightfox7 on November 22, 2014, 10:37:16 pmAn NPN shunt switch would work if standby current draw is not important. However, it appears your application needs least amount of power dissipation for maximum battery life. A PNP switch in this configuration draws no current when not switched on.

author: IanB
author_rank: Super Contributor
timestamp: November 22, 2014, 10:51:21 pm
content: Quote from: nightfox7 on November 22, 2014, 10:40:20 pmYes, PNP.

author: nightfox7
author_rank: Contributor
timestamp: November 22, 2014, 10:53:50 pm
content: Now something I'm getting confused about, almost all of the circuit diagrams show a voltage being applied to the base of the transistor, which I don't think will work with what i'm trying to do.. Right now i'm thoroughly confused..And yes Rory is correct, power consumption is something to take into consideration, I can't have something drawing current needlessly.This is what i'm working with.

author: IanB
author_rank: Super Contributor
timestamp: November 22, 2014, 11:03:15 pm
content: Quote from: nightfox7 on November 22, 2014, 10:53:50 pmYou don't need a voltage on the base of the transistor, but it won't hurt either. Here is a simpler circuit without it.(Edit: corrected error in transistor orientation)

author: rs20
author_rank: Super Contributor
timestamp: November 22, 2014, 11:11:08 pm
content: Quote from: IanB on November 22, 2014, 11:03:15 pmWhy did you vertically flip the transistors in your updated circuit? Isn;t just deleting the resistor fine?

author: IanB
author_rank: Super Contributor
timestamp: November 22, 2014, 11:20:13 pm
content: Quote from: rs20 on November 22, 2014, 11:11:08 pmThat was a drawing error. I did not mean to flip them.

author: nightfox7
author_rank: Contributor
timestamp: November 22, 2014, 11:24:37 pm
content: Sooo.. Like this?

author: rs20
author_rank: Super Contributor
timestamp: November 22, 2014, 11:33:07 pm
content: Quote from: nightfox7 on November 22, 2014, 11:24:37 pmYep, that should work nicely. (BTW, you've basically just made a NOT gate there, you could also use a CMOS NOT gate with a single pullup at the input. But who has those lying around?)

author: IanB
author_rank: Super Contributor
timestamp: November 22, 2014, 11:33:24 pm
content: Quote from: nightfox7 on November 22, 2014, 11:24:37 pmThat should do it.

author: nightfox7
author_rank: Contributor
timestamp: November 23, 2014, 12:01:41 am
content: Awesome, thanks guys.Now, would this be the same thing with a npn?

author: IanB
author_rank: Super Contributor
timestamp: November 23, 2014, 12:19:06 am
content: Quote from: nightfox7 on November 23, 2014, 12:01:41 amNo, but you could try something like this:

author: nightfox7
author_rank: Contributor
timestamp: November 23, 2014, 12:38:40 am
content: Quote from: IanB on November 23, 2014, 12:19:06 amTHIS WORKS.thanks so much, i'll post back here when i get the final thing all soldered up, so y'all can at least see what you were helping me with

author: rs20
author_rank: Super Contributor
timestamp: November 23, 2014, 12:45:05 am
content: As mentioned earlier, the NPN version of this circuit consumes about 1mA all the time, while the input is floating. The PNP version consumes virtually nothing when the input is floating. Just something to keep in mind if there are batteries involved?

author: nightfox7
author_rank: Contributor
timestamp: November 23, 2014, 01:01:38 am
content: Quote from: rs20 on November 23, 2014, 12:45:05 amI plan on building the PNP version as there are fewer components (smaller board, less weight), and because of the power consumption issue.I am just super happy to see it doing what I want, and I've learned a lot..

author: sv2hqx
author_rank: Contributor
timestamp: November 26, 2014, 09:30:44 am
content: I think you should put a 5 volt relay with low current or an 7805You find them both on old routers73Sent from my Optimus 4X HD using Tapatalk

author: nightfox7
author_rank: Contributor
timestamp: December 03, 2014, 04:30:48 am
content: Quote from: sv2hqx on November 26, 2014, 09:30:44 amI didnt know if there would be any weird kickback with a relay..I am using a 7805 to regulate voltage.73s-Seth

author: rs20
author_rank: Super Contributor
timestamp: December 03, 2014, 07:21:15 am
content: Quote from: sv2hqx on November 26, 2014, 09:30:44 amWhat on earth? In what world are a 5 volt relay and a 7805 voltage regulator interchangeable alternatives?