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76  Using Arduino / General Electronics / Re: Tricking a car radio aux input? on: May 07, 2012, 03:47:40 pm
Yeh, you will need to do some experimenting, but putting out a very low signal / very high signal in Hz should fool it and not generate noise (pick outside of the human hearing range).
What might really be happening is the device is not outputting during the period of silence (i.e. the headphones are actually going 'off').

Check it with an oscilloscope to be sure (or some other device that outputs audio via headphones and doesn't suffer the issue..

Might also pay to read if the stereo can disable that feature.
77  Using Arduino / General Electronics / Re: Dummy question about voltmeter on: May 07, 2012, 03:42:49 pm
Measure a AA battery. 1.5V DC should be 1.5V DC.
78  Using Arduino / General Electronics / Re: Unity Gain Amplifier / Voltage Follower - Selecting resistor on: May 06, 2012, 03:23:28 am
No impact at all..

So does that mean the arduino and cable has a higher resistance/impedance than the jumper wires / fingers, etc that were the way?
Doesn't make sense at the moment, in a resistor divider, you have one resistor (the sensor), and the other a known value and you read the mid point of this.

What I find is if I read the voltage between the two cables that go to the sensor (i.e. think of a thermistor, you would wire it up with one side to the ground, and the other to the resistor, and measure the mid point of what you get back).

I wasn't expecting to find the same voltage between the two though - if anything, I'd have expected to find the 5V being supplied to the thermistor.
The diode on the ground side of the sensor is odd too.

Is there perhaps another reason for it to not care ?

I do recall that the controller reset / had randomness when I tried measuring once a while back, but I think I tried measuring resistance of the thermistors in circuit - this would mean I am taking current from the circuit, right?
79  Using Arduino / General Electronics / Re: Unity Gain Amplifier / Voltage Follower - Selecting resistor on: May 05, 2012, 04:27:14 pm
It sounds to me that the temperature sensor arrangement has quite a low source resistance, so you can connect it directly. Can you provide a link to the datasheet for the sensor?

If it's on a long lead then you might want to include a 10K resistor between the sensor and the analog input pin, and a 0.1uF capacitor between the input pin and ground.

Nah no datasheet for it - though it's just a NTC 25 deg C 10k sensor.

Long lead --- from the sensor to the controller - yes, from the controller to my controller? Not so much

Can't hurt to have 10k resistor and the cap - so I'll add that.

Will test it today, connect the arduino direct and measure sensor voltage before and after connection, with any luck it won't change much at all..
80  Using Arduino / General Electronics / Re: Unity Gain Amplifier / Voltage Follower - Selecting resistor on: May 05, 2012, 08:36:29 am
For the purpose of this discussion impedance means the same thing as resistance.

What's needed is a comparatively low impedance connection to the analog in, that way practically all of the voltage is across the high impedance.
If you have a 1000Ω resistor in series with a 100MΩ resistor, practically all of the applied voltage is where?  It is across the 100MΩ, 1000Ω : 100MΩ.

You missed the part where I said to run the op-amp from "Vin", but you can't do that if you're stuck on running from 5V.

I caught that - VIN will be empty - the plan is to not use the voltage regulator and bring in +5 from the already regulated +5 at the other controller - (saves getting an additional supply and then wasted energy from that).

I *was* going to go and chuck a 9V supply in, but if I'm not going to have an issue wiring the input to the arduino (i.e. I don't want the arduino loading down the sensor), then I won't worry about the Op-Amp.
The entire point of the op-amp is so I don't load it down - if the analog inputs are indeed 100Mohm then I would hope that's fine? The sensor reading won't get loaded down, it'll be accurate and I'll get on with reading the voltage...
81  Using Arduino / General Electronics / Re: Unity Gain Amplifier / Voltage Follower - Selecting resistor on: May 05, 2012, 08:19:23 am
Heh, I was thinking of that - the multimeter doesn't affect it - I can go to the analog pin of the MCU being monitored, and the - (gnd) of the sensor, and get a reading - it doesn't appear to drop it at all.

It appears the temperature sensor on the circuit being monitored has a diode on the grnd (looks like a shottky so I think -- lightning protection?)
And the positive is a 10k resistor.

Is the analog inputs really in the megaohms of resistance though?
How can I make my connection high impedance ? Does putting a 1Mohm resistor at the connecting point accomplish this (i.e. stops it drawing current?) 

If I connect, I'll be loading down the temperature circuit (if it's wrong and the impedance isn't greater in the duino), I think that'll just cause it to think the sensor is 'hotter' than it is, until the current draw stops (as current increases, voltage decreases, so the temperature reading to the controller would be 'hotter').
82  Using Arduino / General Electronics / Re: Unity Gain Amplifier / Voltage Follower - Selecting resistor on: May 05, 2012, 02:42:56 am
Oh I see - Most of the Op Amps can only produce up to (VCC - 1.5V).
To get the full 5V - I need a supply of 6.5+ Vcc.

What are the downsides of using a DC-DC to get 6.5 from 5Vdc ?

5Vdc from the existing controller (mains powered), to the arduino 5Vdc, and then take from the 5Vdc to a DC-DC converter to get 7V with minimal power (OpAmp = low power).
83  Using Arduino / General Electronics / Re: Unity Gain Amplifier / Voltage Follower - Selecting resistor on: May 05, 2012, 12:58:25 am
LM358 might work, but I think rail-to-rail would be better (Gnd to 5V on input = Gnd to 5V on output).

That 3.5 - 5V would equal just below 21 deg in temperature - don't want to lose that much of the range.
84  Using Arduino / General Electronics / Re: Unity Gain Amplifier / Voltage Follower - Selecting resistor on: May 04, 2012, 11:22:45 pm
Ahh,  VCC(5V) - 1.5V = 3.5V (so I can read from 0 to 3.5V)..

I thought of LM358 (due to how common it is), but can't find in the datasheet where it is high impedance (i.e. like the JFET input in the TL072).

I need the high impedance so that we don't take current from the circuit being monitored.
85  Using Arduino / General Electronics / Re: Unity Gain Amplifier / Voltage Follower - Selecting resistor on: May 04, 2012, 09:18:45 pm
Another thought is some sort of voltmeter circuit - I need it to be high impedance though - just like a multimeter - so it doesn't interfere with the readings.

The voltmeter would need a 0-5V scale (since the most the sensor can go is 5V and gnd when short cct.

I don't really want to bring a negative supply into the equation, so I think I need a rail-to-rail opamp one that can go right close to ground (the measurements we are interested in will be in the range of 0.7 ? to around 3V when cold. I was reading that Op Amps don't typically output that close (though rail-to rail does?)

And I need it to do that with 5V input and single supply (5V and GND).

TLV2231 ? Bad choice?
86  Using Arduino / General Electronics / Re: Unity Gain Amplifier / Voltage Follower - Selecting resistor on: May 04, 2012, 07:43:16 pm
So I need another component then ?

I want to have the 'no current load' on the circuit being monitored - and ideally want to do it from the same supply as the circuit being monitored (which should be 5V).

Any suggestions on an Op-Amp capable of producing that from +5 only!?

The plan is as "CrossRoads" previously posted here: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1293425807/21#21
Finally making some ground on this - but yeh, he said single power supply there ( comprehension fail!).
87  Using Arduino / General Electronics / Re: Unity Gain Amplifier / Voltage Follower - Selecting resistor on: May 04, 2012, 07:06:58 pm
So, from figure one and the many examples on Google..

Supply to Opamp - 5V from Power Supply.
Ground common to device being monitored.
Monitored Input to Non-inverting + of OpAmp
Input of Non-inverting - of Opamp tied to Output.
Output has no capacitor or resistor as in Figure 1 - it just goes straight to Analog In of arduino?

Sounds too simple?
88  Using Arduino / General Electronics / Unity Gain Amplifier / Voltage Follower - Selecting resistor on: May 04, 2012, 06:34:26 pm
I want to put together a voltage follower, as in the circuit diagram here;
http://www.jaycar.com.au/products_uploaded/ZL3072.pdf
- Figure 1 - Unity Gain Amplifier

Note that there is a capacitor and resistor on the output - if I want no gain (just the exact voltage on Vin (voltage follower), what resistor size do I need?

I can't see it - how do you size the resistor and capacitor in the voltage follower circuit?
89  Using Arduino / General Electronics / Re: ground loop issue on: May 01, 2012, 03:37:47 am
Backfeed to the USB port at the PC, not the supply from the PC to the 'duino..?

Check the true current consumption by getting a multimeter set to mA between the supply and arduino (put it between VIN pin and the supply).
90  Using Arduino / General Electronics / Re: Sharing a thermister on: April 28, 2012, 11:56:47 pm
That was the reason it was suggested to use TL072 - it has a very high impedance so as to not cause interference on the circuit.

With respect to the capacitor - I'm not sure where I should pick up the input from the thermistor?
I can get it from before the resistors on the other side, or after it - in fact I could go as far back as the MCU that's reading it - is that the ideal place to measure from?

What will happen is I'll get the voltage reading and build an array of the voltage to temperature values and that should give me close enough to a reference of the temperature being read by the other controller..

What I think will work is:
Thermistor -> Existing controller.
Then solder to the existing input (either at the thermistor, before the resistors, or after the resistors, at the MCU), cable to bring it out to TL072 Op Amp following the voltage reading.
The TL072 as a voltage follower will feed the same voltage reading into my Arduino and it will reference it's table of voltages to decide what temperature it is.

So, do I get the TL072 input from the + of the thermistor (i.e. the field side of the thermistor), or from the MCU side which will be after the resistor (i.e. the midpoint)?
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