SparkFun Forums

[jimshell21]

Hi-

A newbie here, thanks for considering my question:


I'm involved in a project that will use a linear accelerometer to collect an analog voltage over the course of 10-100 seconds.

I desire to (1) excite the accelerometer via battery power, (2) collect the voltage output and (3) continuously transmit the analog voltage signal via Bluetooth or RF to a receiver connected to a PC/DAQ system for A/D conversion and data processing.

Is there a simple way to do this-ie can the A/D conversion be done on the PC/DAQ system after a signal is sent wirelessly?

Any comments or suggestions are very much welcomed.

Thanks!

[waltr]

Not with Bluetooth as it is a digital media therefore the information must be in a digital format (serial string of bits) to be transmitted.

There are other RF transmission methods that can transmit an analog signal but the accuracy is limited unless extreme care and calibration is done.
There are other methods that are more of a mix. One is the convert the voltage to a frequency, send the frequency over the RF link then either convert back to a voltage and digitize or convert the frequency to a digital value.

[jimshell21]

Hi Waltr-

Thanks for hitting the nail on the head with your helpful reply.

Didn't know Bluetooth was discretized data.

We desire to do exactly what you described "One is the convert the voltage to a frequency, send the frequency over the RF link then either convert back to a voltage and digitize or convert the frequency to a digital value."

We might also need to multiplex data, up to 12 channels.

Can you suggest an appropriate RF method/device to get us started? I realize there must be many to choose from, but a general site link or product name would be very helpful...

Warm regards,
Adam

[waltr]

The 433MHz RF units that Sparkfun sells could work. These are simple OOK modulated RF units. Multiplexing several signals can be done but becomes much more complicated. You could do a combination of frequency and pulse width encoding.

Do some googling on modulation methods, multiplexed RF, analog modulation for starters.
There is a reason that much RF communication has gone digital. Microcontrollers with built in ADCs and UARTs are fast, small, cheap and versatile. Even the frequency and/or pulse width encoding is easier with a uController.

[Vraz]

The other method you might look at is pulse positional modulation (PPM). This is the method that the older R/C transmitters used to send multichannel positional (analog) signals in a non-digital world. Essentially, they use simple binary transmission and the pulse width (ratio of high to low) represents the analog signal. Resolution is a function of the maximum pulse width (the larger it is, the better the resolution, but the lower the sampling rate). The other interesting aspect about PPM is that on the microcontroller side, you don't use a ADC at all-- its purely software to calculate the pulse width.

[SpinDrah]

why do you want to do it this way? you are taking an easy thing, and making it very difficult.

[theatrus]

If you've considered using Bluetooth, you are already using a microcontroller. Its hard to locate a microcontroller which does not have an on-board A/D unit, therefore you're making an easy solution very difficult.

Yes, you can send analog frequency data over RF. However, its very hard to tune. The simplest approach is AM modulation.

[jimshell21]

thanks for all the very helpful replies-we're on our way!

[QuantumKid]

This is old now, but I am trying to do essentially the same thing for a project but have yet to find a good solution. I need to read analog sensor data and transmit it wirelessly to another circuit. Ideally I don't want to go through any ADCs or DACs because of latency. Sure, Xbees and bluetooth stuff are technically fast, but even 1-10ms of latency will cause my system to fail, and with an analog read - convert to serial - transmit via serial - write to analog protocol (using two Redboards, two Xbees, and an analog shield I found for the DAC) there is about 100ms of latency which makes this approach hopeless. I need to be able to respond to these analog readings (error signals) in order to servo a device on the other end, and even 1ms of latency would cause my system to run at a snails pace (and ultimately be pretty useless).

Thus the need for a truly analog RF transmission, so that the only significant latency comes from the RF circuitry which should be far less than 1ms. I can filter, amplify and shift the signal levels with analog circuitry before and after the RF link, so I am not too worried about noise at the moment (unless it's really bad). Does anyone have a suggestion of a direct analog voltage to AM or FM radio signal method? I only need the link to go one way too, btw. No need to multiplex if I can setup at least 3 distinct links.

Any advice or suggestion would be much appreciated.

Thanks!

[jremington]

Voice/music transmission over commerical AM and FM radio is analog.
Sending DC values is usually done by generation of an audio subcarrier.

[lyndon]

The classic way to do this was with voltage-to-frequency conversion then use the frequency to modulate a transmitter and do frequency-to-voltage transmission at the other end. There are chips that will make it easier, but in the end it's an obsolete technology for a reason.

I'm inclined to say that if you are having latency problems doing it digitally, then you're doing something wrong (but I don't know what!). Once your receiver & transmitter are communicating, achieving 1ms latency with digital communication should be trivial.