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31  Using Arduino / General Electronics / Re: Problem using a voltage divider to input a voltage to a low impedance line on: July 28, 2012, 03:22:21 pm
Since all you want to do is drive a low impedance load using PWM, why not use a simple FET driver circuit as per this arduino tutorial
http://www.arduino.cc/playground/Learning/SolenoidTutorial

The "load" I have is just one signal wire going into the device. Although they share a common ground, I can't connect the load up in series. Is there any way I can use a FET in that configuration? That's what I'm struggling with.

(Thanks for the reference to the tutorial, by the way. I'm just learning about transistors, and that tutorial is a helpful one I had not seen.)
32  Using Arduino / General Electronics / Re: Problem using a voltage divider to input a voltage to a low impedance line on: July 28, 2012, 01:01:10 pm
A transistor used in emitter follower mode has the "load" between emitter and ground.  Therefore "all" of the output voltage appears across this load.  There must always be a drive voltage between base and emitter to produce the base-to-emitter drive current.  Hence the emitter voltage can never be greater than the base voltage (both relative to ground).   If the base voltage is the PWM output of the arduino - a nominal 5 volts, it follows that the emitter voltage can never be greater than the arduino voltage. 

Thanks for the explanation. That makes sense. It's nice to understand why I was seeing what I was seeing. It had me stumped, and I had not yet found an explanation elsewhere.

Now the question is--how can I get the 0 to 12 Volt range that I want? Obviously it is not going to be in the common emitter mode. I'll keep looking for an answer. Any help will be appreciated.

Quote
I need to send a signal of between 0 and 12 Volt to an electric-over-hydraulic trailer brake.
I stumbled, why not put simple question in subject line "how to switch my trailer brake?"
 Low impedance line ?   

I didn't mention what the load was because I did not know that information was important. Plus I did not want to spark a lot of discussion along the lines of: "Just use a commercial in-cab brake controller." or "It's dangerous and stupid to use a homemade brake controller on the public roads. You'll kill us all!"
33  Using Arduino / General Electronics / Re: Problem using a voltage divider to input a voltage to a low impedance line on: July 27, 2012, 09:26:59 pm
Well, I'm clearly not understanding something here.

I tried the TIP120 transistor (a Darlington) using 0 to 5 Volts instead of 0 to 12 Volts. The highest output I got was 3.7 Volts. Then I tried using a MOSFET and the highest output was less, about 3.0 Volts. I can't figure out why I cannot get the full 5 Volts.

What am I missing?

UPDATE: This situation is about what Jackrae said in post #2.
This will limit output signal to around 4.3 volts
But I do not understand why the voltage is limited. I tried using 12 Volts as the supply (rather than 5 Volts) and got about the same results.
34  Using Arduino / General Electronics / Re: Problem using a voltage divider to input a voltage to a low impedance line on: July 27, 2012, 08:41:01 pm
What is the load?

I need to send a signal of between 0 and 12 Volt to an electric-over-hydraulic trailer brake.
35  Using Arduino / General Electronics / Re: Problem using a voltage divider to input a voltage to a low impedance line on: July 27, 2012, 07:47:12 pm
I decided to give the TIP120 transistor a try. It looks like it should work. It does have a voltage drop of about 1.3 Volts. But it gives a nice voltage range without the big jump I had when using the potentiometer.

I'm got another circuit with a MOSFET that I think I will build and try out. That may give me close to the full 0 to 12 Volt range.
36  Using Arduino / General Electronics / Re: Problem using a voltage divider to input a voltage to a low impedance line on: July 24, 2012, 11:52:38 am
Thanks for the suggestions. I'll look into them. One thing is that I do not need to filter the output to get rid of the PWM. The output signal line can take PWM fine.
37  Using Arduino / General Electronics / Re: Problem using a voltage divider to input a voltage to a low impedance line on: July 24, 2012, 11:34:26 am
how "low" impedance? 500? 50 ? 1 Ohm? how "fast" voltage has to be set?

About 50 Ohms. Voltage does not have to be set quickly.
38  Using Arduino / General Electronics / Re: Problem using a voltage divider to input a voltage to a low impedance line on: July 24, 2012, 11:13:32 am
Thanks for the reply. I need to get the full 0 to 12 Volt range on the output line. Is there any way to do that?
39  Using Arduino / General Electronics / Problem using a voltage divider to input a voltage to a low impedance line on: July 23, 2012, 08:39:01 pm
I had a problem using a voltage divider to input a voltage (between 0 and 12 volts) to a low impedance signal line. I used a potentiometer as the voltage divider, and as I turned the pot the signal voltage would suddenly jump from around 1 volt to over 10 volts. So that did not work.

My new plan is to use a TIP120 transistor to send the voltage signal. I'll use an Arduino analog PWM output to the transistor's gate, and put ground and +12 volts through the transistor. By varying the output from the Arduino from 0 to 255, I hope to get the whole range between 0 and 12 volts. 

Will that give a good voltage signal on the low impedance signal line?
40  Using Arduino / General Electronics / Re: Where can I buy RJ45 female connectors? on: July 12, 2012, 10:51:51 am
Thanks, MarkT. You are right. At Digikey I searched for USB receptacle, and there they were. It's nice to learn to be posher.
41  Using Arduino / General Electronics / Re: Where can I buy RJ45 female connectors? on: July 11, 2012, 10:07:49 pm
Oops, my mistake. I said RJ-45 female connectors, but what I was having trouble finding was USB female connectors. Jameco has them. Digikey and Mouser do not seem to.

And keep in mind some have integrated isolation transformers ("magnetics") that you'll want to avoid.

Thanks for the heads-up about the magnetics. I wondered what that meant.
42  Using Arduino / General Electronics / Re: Where can I buy RJ45 female connectors? on: July 11, 2012, 08:22:02 pm
Found them at Jameco. I knew that would happen. Whenever I give up and post, I find the answer shortly thereafter.
43  Using Arduino / General Electronics / Where can I buy RJ45 female connectors? on: July 11, 2012, 08:06:34 pm
I'm working on a project where I use Cat5e cable to send four digital signals from an Arduino to some electronics about six feet away. I have the Cat5e cable that I need, with a male connector on one end.

What I need, though, is the RJ45 female connector to go with the electronics. I want to mount that connector on perfboard and run wires from the connector to some transistors and other components.

I thought those connectors would be easy to find, but can't find them anywhere. Any suggestions on where to look?
44  Using Arduino / Project Guidance / Re: Ok, I finished an Arduino project. How to make it permanent? on: May 14, 2012, 01:13:06 am
Let me second Coding Badly's suggestion of the Teensy. They cost $16 but they work really well if you need the serial over USB function. The other option I have used to buy a cheap USB programmer off of eBay and use it with an RBBB (Really Bare Bones Board) Arduino. That ended up costing about the same.

Adafruit sells a board for $20 that is similar to the Teensy. http://www.adafruit.com/products/296 But I could not get that board to work with serial over USB. (Plus I had a real tough time programming that board. The Teensy was much better.)
45  Using Arduino / Project Guidance / Re: Motorcycle loom on: May 04, 2012, 12:47:45 pm
That's an interesting project. We converted an old pickup to an electric car, and replaced the wiring harness with a network-type setup. We use a Teensy (not an official Arduino, but like one) at each "corner," and connect the lights, horn and other accessories through a 4-relay board that is switched by the Teensy. It works well, replacing a complicated and heavy wiring harness with a simple network.

On a motorcycle, it might not work as well. Especially if you have all the wiring for the gasoline engine as well. You probably would not get the simplicity we can get, and since the distances are shorter on a motorcycle, the cost-benefit ration may not be in your favor.

Certainly it would take some effort. But it could be done, and you would learn a lot if nothing else. Whether you want to do it or not, only you can decide.
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