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15106  Forum 2005-2010 (read only) / Interfacing / Re: ID-12 on: February 08, 2009, 08:30:51 pm
"Does the FT232 still transmit data to the ATmega 168 after a sketch has been uploaded and the board has been disconnected from the computer? "

 No it doesn't transmit any data, but it's chip is still powered up by the same 5vdc that powers the rest of the board so it's TTL output pin going to the Arduino's serial input pin is still electrically active, most likely sending a steady HIGH signal. If you could somehow not hook your external serial signal to the Arduino until after you disconnect the USB cable and then connect it, I think that might work, but again you are kind of on your own as it's untested by me and others would have to be sought out if it has worked for them.

15107  Forum 2005-2010 (read only) / Interfacing / Re: ID-12 on: February 08, 2009, 08:03:42 pm
Well the Arduino hardware send and receive pins are 'hardwired" on the board to the USB serial converter chip. That's not necessarily a problem for the Arduino send data pin as sending to two input devices is electrically OK, however it's a problem for the Arduino input pin as an external signal will be trying to 'fight' with the output signal from the USB chip. There are two series resistors between the USB signals and the Arduino serial pins so I don't think there would be any permanent damage, and I've never tried to do that (share the hardware serial port with two devices) but it seems from other posting of problems and advice that it is not a good solution.

 Anyway going to softserial eliminates the problem by then using physically different pins that have no other electrical connections to anything else.

15108  Forum 2005-2010 (read only) / Interfacing / Re: Resistive sensors? on: February 08, 2009, 06:00:26 pm
Could you please explain how you came up with the values, I would like to understand how to do it, rather than relying on the community
It's just using iterations of ohm's law. You calculate the the total series resistance of the sensor (at one extreme range value) and the fixed resistor by addition. Then you calc the series current by I = E(5vdc) divided by the total series resistance. Then calculate the voltage drop across the sensor resistance by multiplying E = I X R. You have to do the same calculation for when the sensor is at it's other extreme value. And of course because you have sensor(s) with different values you have to do the whole thing for the other sensors values. Making the fixed resistors approx equal the the maximum sensor's resistance gives you a useful swing in output voltage range.

What we have here is a basic voltage divider network with one leg (the sensor) that is variable resistance.

15109  Forum 2005-2010 (read only) / Interfacing / Re: Resistive sensors? on: February 08, 2009, 05:33:57 pm
For the 0-90 ohm sensor(s) if you use a 100 ohm fixed resistor the resulting analog voltage measured will range from 0 volts up to 2.37  volts. For the 54-1254 sensor(s) a 1,000 ohm fixed resistor will measure from .26 volts to 2.78 volts. You can scale/correct these ranges in software once you know the raw counts of range.

What kind of sensors are these anyway?

Good luck

15110  Forum 2005-2010 (read only) / Interfacing / Re: Resistive sensors? on: February 08, 2009, 05:02:14 pm
Not quite. Each individual sensor wire will have to have it's own series resistor wired to +5vdc. The junction of the sensor wire and the new resistor will wire to an individual analog input pin.

The size of the new resistors depends on the range of minimum to maximum resistance of your sensor. If you don't have a data sheet showing this range of resistance change, you will have to use a ohm meter and manipulate the senser input and read the ohm value change for the range desired.

15111  Forum 2005-2010 (read only) / Interfacing / Re: 24V DC Solonoid on: February 08, 2009, 01:58:42 pm
You didn't say how much current your 24vdc solenoids draw nor how many solenoids you will be using nor how many might be powered on at any given time. All that info is needed to determine the total amount of 24v current that will be needed.

 I suspect that the DC/DC convertor you show won't do the job as one watt at 24vdc is only 42ma. I don't think there is a simple solution other then a properly sized 24vdc supply.

15112  Forum 2005-2010 (read only) / Interfacing / Re: How bad is negative voltage on analog input? on: February 07, 2009, 02:34:14 am
Any voltage less then around -5vdc will cause an internal protection clamping diode to start conducting current, and as the voltage lowers more there will be more current drawn until the diode is damaged.

The negative voltage won't harm the pin if you can externally limit the amount of current that can be drawn through the forward conducting diode. If you wire a series resistor from the pin to your voltage to be measured then there will be no harm when your measurement voltage goes negative. A 2K resistor should be OK.

15113  Forum 2005-2010 (read only) / Interfacing / Re: Transient filter dimensioning on: February 05, 2009, 04:51:29 pm
Rather then a cap (sizing will be tricky and only testing will find an exceptable value I think) how about just wiring a 5 watt 5.6 volt zener across the output of the wall wart power module?

15114  Forum 2005-2010 (read only) / Interfacing / Re: BREADBOARDS?????? on: January 31, 2009, 08:30:23 pm
"Will I still only need the supporting pull-up and crystal?"

If you use a crystal you need two caps, however the three termial resonator has caps internal. You may want a small push button reset switch, however just power off and on the breadboard will restart the program. Of course your breadboard needs to have a good regulated 5vdc power source for the micro and it's good practice to install a small .1mfd cap between the +5vdc pin and ground mounted close to the micro chip.

Here is a module I bought recently that mounts to a breadboard but has the minimum needed support parts. It comes with a 168 chip with the arduino bootloader already in it. It saves some breadboarding wiring and makes the module more portable. Well worth $10.

15115  Forum 2005-2010 (read only) / Interfacing / Re: BREADBOARDS?????? on: January 31, 2009, 08:02:54 pm
Yes for the arduino system it is certainly possible to preprogram a sketch into the chip while it's in the arduino board, then remove the chip to a breadboard. The only support the chip will need on the breadboard is a pull up resistor for the reset pin and a crystal or resonator.

There are several clone arduino suppliers that have made modules that plug into a breadboard and that would save you having to change chips.


15116  Forum 2005-2010 (read only) / Interfacing / Re: Working with a Microphone on: January 31, 2009, 07:05:01 pm
Yes, if you are going to be playing around with a Arduion you NEED to get a digital multimeter. They don't cost much these days and I can't see how anyone could get along without on, even you software types  smiley-wink


15117  Forum 2005-2010 (read only) / Interfacing / Re: Using same I/O pin for button input & LED output on: February 01, 2009, 05:05:08 am
Also be careful from a hardware perspective if you attempt this dual use. If the button was wired to +5 or ground on one end and it was still pressed down when your program switches the pin to output mode, then you would short circuit the output pin and burn it out.

So be sure to add a series resistor on one side of the switch such that a output pin would not sink or source more then 20ma or so if the switch was held down.


15118  Forum 2005-2010 (read only) / Interfacing / Re: Has anyone seen/used this little gem? on: January 31, 2009, 01:59:25 am
Have not tried them. It looks good, however it states in the data sheet if one needs true full duplex capablity you need to use two of these. Not sure if the Arduino IDE uses full duplex or half duplex communications.


15119  Forum 2005-2010 (read only) / Interfacing / Re: Wiring a Condenser Mic for use with arduino on: January 31, 2009, 08:14:48 pm
"Also, does the arduino output DC or AC (actually, this is probably on the arduino data sheet, Ill have to check that)."

That is a problem in that microprocessors only handle positive voltages from 0 to +5vdc. So after amplifying a audio signal such that it's peak to peak voltage is 5volt, it has to be injected with a positive +2.5volt voltage to center it to the micros A/D input range. This can be done with either a voltage divider or another op amp.

 Also I think you will find that the Arduino will not be able to sample a audio signal and be able to recover any useful data, it's just doesn't have the horsepower to do it directly. Off chip modules would be required to digitize a audio system and other off chip modules needed to store the large data streams that would result. At best the Arduino would be 'central control' directing the flow on analog and digital streams to other dedicated hardware/firmware modules.


15120  Forum 2005-2010 (read only) / Interfacing / Re: Wiring a Condenser Mic for use with arduino on: January 31, 2009, 07:18:56 pm

That wiki shows a typical input circuit drawing using one of these mics.
The DC voltage source wired through a resistor supplies a smallbias current ( a few milliamps) need for the internal FET transistor these mics have. The capacitor shown blocks the DC voltage and lets only the AC audio voltages through to the rest of the circuit.

The thing is these mics only output a few millivolts AC audio signal and need lots of amplification to raise to useful levels. An Arduino A/D input pin is not going to show much activity without amplification. Maybe if you could explain what you are trying to accomplish we could be of better assistance to you.

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