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121  Using Arduino / General Electronics / Capacitor Sizing on: November 24, 2011, 03:25:12 am
Tried to google, but can't make sense of Amps to Farads.

I want to have an arduino board remain powered on for around 10 minutes after power is lost. The function is in a car circuit where the power will only be available when the ignition is on, and once lost, the arduino should have around 5 - 10 minutes to switch off.

I thought of doing it with a relay driving it from the arduino to get it to turn itself off, but thought a capacitor would be a better idea.

The arduino board and it's regulator seem to suck down around 100mA with the 12V hooked up (the reason I'm not connecting the battery side to it!) - so the capacitor solution should give me a good few minutes power and stop any waste of battery power!

But - how do I pick a capacitor up to the task?
122  Using Arduino / General Electronics / Re: Send a "0V" signal? (Ground?) on: July 26, 2011, 09:09:37 am
A transistor is what I'd need to ground the signal (i.e. after I know an event has occurred), but what about the connection to the interrupt pin?

A diode and resistor to ensure voltage and current remain acceptable I suppose.

What happens at the transistor base if it's connected to an arduino input pin?
I've had a look online for this specific question but could not get an answer, would it being open, cause the transistor to be fully open, or is there still some leakage (I'm just trying to get ahead, as it will be sharing the same connection at the control unit, some leakage could cause intermittent issues that I'd like to get ahead of).

- A NPN transistor C - to the control unit pin (which waits for the signal to be grounded to decide it needs to unlock / lock all doors), B- Arduino pin set to input mode when not needed, set to output when I need to ground the signal (assuming input is the right pinmode for this!), E- Ground
- A Diode and Resistor on the control unit pin that gets the grounded signal from the lock, to interrupt Pin 0 / 1.

Then in the code, interrupt will wait for event (such as ignition in on position), if so, issue lock all doors by grounding the pin that would be grounded if the drivers side door was locked.
If key just reached acc position, unlock all doors.
Or if key not in ignition, 300 seconds, lock doors.

I'm mostly concerned about interference by joining onto the existing cabling that goes into the control unit (as logic is easily corrected), so I hope I have my bases covered, but would appreciate any further tips with respect to joining onto the pins (the ones I will be are the pins that get grounded by the lock so I can ground those to get the desired lock / unlock actions without interfering with it's normal operation) and the ignition 'key present' indication (it has a beep alarm that reminds you to remove the key, so it's electrically possible to know there is a key there, I believe this will be 12V - so a zener diode at 5.1V is what I thought here?)
123  Using Arduino / General Electronics / Re: Send a "0V" signal? (Ground?) on: July 26, 2011, 03:50:46 am
Purely out of interest purposes, how is the voltage out of the arduino's range, when it is in fact 5V?
I'll put the transistor in, but just curious how that is so?

Is it because of the potential for a fault to develop, and  >5V could end up there?
From the PDF, it's 5V, and I was of the understanding Atmega328's tolerate input of 5V.
124  Using Arduino / General Electronics / Re: Send a "0V" signal? (Ground?) on: July 25, 2011, 02:11:02 am
The PDF I linked in the opening post shows the signal as 5V neutral.
It won't be current, as I think it's just a signal to their control unit - but  if an NPN transistor to ground accomplishes the same effect without any negative results - it's probably worth doing.

There shouldn't be any voltages on the pins higher than the 5V neutral that the signal across the lock has - I doubt much current is drawn.
125  Using Arduino / General Electronics / Re: Send a "0V" signal? (Ground?) on: July 24, 2011, 07:43:35 am
I've got the how - digitalWrite(pin, LOW) to ground the pin, configured as an output.

But I'm still not sure if it'll be electrically safe to simply run the cable to the pin (i.e. what would happen if the lock was turned, and the other wire was left at it's HIGH state - wouldn't that stop it working)?
Not sure if I need to do any isolation in terms of resistors or the like to make sure it's safe to connect with?

Would the solution then be to configure the pin as an input after use - so that it's not going to interrupt the central locking from it's usual operation, such as setting it high (5V) would by making the system think there is no key turning the lock...?
126  Using Arduino / General Electronics / Send a "0V" signal? (Ground?) on: July 24, 2011, 06:02:07 am
A dumb question, but I need to know.

I've got a car with central locking, the problem is I want to add 3 features to it using an arduino.
I know that Pin 23 is the "LOCK" signal, and PIN "35" is the unlock signal.

I will be able to use an interrupt to determine 12V is present on one of the interrupt pins.

The document says that in 'Neutral' (i.e. no lock or unlock signal), there is 5V present on pins 23 and 35. When a signal is sent,  it will be at 0V - so to get them to lock / unlock, I plan to join wires to the existing cabling at those pins, and then have arduino provide the ground source for it based on ignition signal.

What I don't know is how can I ground the pin using arduino to deliver the necessary 0V signal ?
Is there anything I should watch out for in doing this - Am I right in joining on to the in place circuit (at the pin, there will be two wires leaving, one to arduino, one to the locks / switch  - do I need to isolate that at all?

Some more info if it helps!:
Page 17 of the PDF describes the involvement of the pins and the locks (5V neutral, 0V when key turned).

I plan to have the "ON" position of the ignition through a zener diode, and use that as an interrupt to indicate the ignition is on. When the ignition goes to "ON", a lock signal should be sent. When it goes to "Acc" or "OFF", an unlock signal should be sent (so a "CHANGE" interrupt should do it).
I might then go further to have the key switch (detects key present in barrel), to then have a interrupt timed to send a lock signal.

Anything to watch out for with my plan?
127  Using Arduino / Project Guidance / Float Usage on: February 07, 2011, 04:42:53 am
I've read the documentation, I can see here:

A few issues are noted:
- The document notes that floating point math is slower. By how much? How many in each second?
- 6.0 / 3.0 may not equal 2.0 ? How so?
128  Forum 2005-2010 (read only) / Interfacing / Re: interfacing with a cheap RC car? on: December 29, 2010, 09:29:55 pm
Very very easy.
Take the ground, connect to arduino.
Connect the positive to the Arduino pin 7.

Control the speed of the motor using PWM -
Full steam ahead: analogWrite(pin7, 255)
Slow down (i.e. if a sensor detected it was approaching an object): analogWrite(pin7, 90);

analogWrite(pin7, 0);


The above would work if the motor was a 5V motor.
129  Forum 2005-2010 (read only) / Interfacing / Re: Thermistor connection on: December 29, 2010, 10:04:11 pm
Good point.

I put it between the + and - from the thermistor with the cable in circuit.

So, the controller had the sensor cabling all connected, powered up as normal, etc.
I took the meter, black on the - and red on the +, and red 1.36V - is this flawed in some way?

Do I take the - from the thermistor, and send it to the ground, and + to the input of the op amp, or will that not work?

I think that would work - it would eventually create a connection between the ground that terminal provides to the multimeter as an example, as opposed to running 3 cables two of which are ground..
130  Forum 2005-2010 (read only) / Interfacing / Re: Thermistor connection on: December 29, 2010, 06:03:23 pm
Wire it up like figure 1, make sure you get the ground of the two systems connected.

When you say get the ground of the two connected.. the controller's DC ground, needs to be connected to the ground of my circuit, or is it the - cable from the thermistor, can go to the ground for the circuit?

i.e. Signal ground from the thermistor to the ground of the arduino, or DC ground of the controller, to ground of the arduino ?

Why does that matter if I'm reading voltage - i.e. the multimeter never needed that?
131  Forum 2005-2010 (read only) / Interfacing / Re: Thermistor connection on: December 29, 2010, 08:15:26 am
I was just revisiting the relay idea - the resistor that is at 10k - this would affect the readings the controller received right?

i.e. if I put 5V across that 10k resistor, it would think the temperature was the same.

So, if my circuit came live, with the pump running, the controller would get two of the same values for that mini second the resistor was showing, and turn the pump off...

I think I want to try:
- Pulling the "NEXT" button on the circuit low - using the arduino.
- Monitor the output from the 7-segment display driver, to read the numbers put out..

This would remove interference to the pump controlling aspects of the circuit as the Next and LEDs aren't used in the monitoring.
What do I need to do make that concept possible?

Is it as easy as removing the 'next' button, applying the 5V from the arduino to the next button, and then monitoring the voltage output to the 7-segment displays - there's 3 of them, and the values can be "SSd", or numbers.

I'm already also looking to monitor pump on / off activity by monitoring the voltage through the pump on LED.

There's 3 segment displays, I guess they are driven by chip - M74HC238 (only one that looks like it isn't controlling 240V relay, and isn't the PIC microcontroller).

Am I off the beaten track or is the above a better idea?

I'm happy to get photos of the circuit if that will help design one (I've got near 0 design abilities when it comes to anything other than connecting voltage to something!)...

Which would you do? Monitor the pump LED to see if it was on, then hold off until it went off?
Pull NEXT button low, read resulting output to segment displays?
132  Forum 2005-2010 (read only) / Interfacing / Re: Thermistor connection on: December 28, 2010, 02:22:37 am
You could power down your system and take some measurements with an ohmmeter, will tell you the range of resister you needed.

I know it's a 10k thermistor - so I assume a 10k resistor is needed.

How would that help out tho? We can hook up a 10k resistor from... to.. ?

I'm looking into the op amp buffer at the moment as I like the concept of it - from what I gather, I attach a wire to the positive side of the thermistor, then send this to the positive of the op amp.

I'm testing this concept with a AA battery and an LM358 at the moment, it takes 1.38V and turns it into 0.15V.
133  Forum 2005-2010 (read only) / Interfacing / Re: Thermistor connection on: December 28, 2010, 01:22:44 am
I can get inside the controller (I've been in there to hook up my cabling to get the readings).

I'll look into the op-amp buffer, If I'm not mistaken this sounds like it would replicate the data to the arduino ADC without impacting the resistance to the controller's own MCU.

And that would beat trying to get the data from the MCU in the controller.
134  Forum 2005-2010 (read only) / Interfacing / Re: Thermistor connection on: December 28, 2010, 01:19:59 am
Heh, that was the very first idea I had!

Faking the resistance to the controller, so I could read the thermistors directly.

I couldn't find anyway of doing that.

I could use a digital pot apparently, but the controller has resistors too, and I don't want the risk of it not having it's data.

Speaking of that... will the relay default back to the controller in the event the batteries are flat?
135  Forum 2005-2010 (read only) / Interfacing / Re: Thermistor connection on: December 28, 2010, 12:36:10 am
I'll have to try the relay idea when I can.

I tried to use the arduino to read the voltages, but I turned the arduino off to see what happens if the battery is flat, this caused the readings to jump to wrong values and therefore would not serve the purpose I need.

I should add - the purpose of this is to monitor the temperatures of a solar hot water collector and water tank, I mainly need to know if it's been above 60oC at least once every 3 days, but also, would like to have the data logged too.

I've thought of using Dallas sensors on the pipes, but this was inaccurate, measuring the voltage outcome from the sensor circuit isn't going to work.

Adding another thermistor isn't going to work as there's no room to.

So this leaves really, checking to see if the relay will work without causing the pump to kick in and out (I think that will be an issue).

Hack the PIC microcontroller on the controller unit to give the data to the arduino, then go wireless from there. I don't know how PIC is coded or pin outs...

Make my own unit to do exactly what it does (monitors the thermistors, if there's a 12 degree difference, kick the pump in, whilst the tank is under a set value). The problem with this is I'd have to dig up a lot of information to get it done right.

The unit by the way is the "SolaStat-ST" controller, it's not the "Plus" version that has RS485 (which would make this so much easier).
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