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1  Using Arduino / Project Guidance / RFID for car detection on: July 02, 2014, 09:54:02 am
I'm working on a research project where I have to align a car with an RFID tag and reader. The reader is on the ground and the tag is on the bottom of the car. The distance between the car and the ground is approximately 25 cm.

I need a compact and inexpensive solution. I have looked into the 125kHz systems, but the antennas are huge and expensive with that range. I have also looked into 13.56MHz systems, but all the readers and antennas I found are expensive. For UHF, I'm worried about signal distortion because of all the metal beneath the car.

Do you know of any inexpensive solutions in the range of $10 to $50? Should I go with a DIY approach? All the DIY solutions I have read have huge ass antennas and I need something that is quite compact.
2  Using Arduino / General Electronics / Can someone identify this symbol in the Arduino schematic? on: August 30, 2011, 03:59:23 pm
Does anyone know what this symbol is?

3  Using Arduino / Audio / Re: Is this an appropriate circuit to drive my 8 ohm speaker using PWM? on: March 03, 2011, 03:08:46 pm
This one:

4  Using Arduino / Audio / Re: Is this an appropriate circuit to drive my 8 ohm speaker using PWM? on: March 03, 2011, 03:00:52 pm
I think I figured it out, since with 0 ohms at the pot, the voltage output of the opamp would be 1V, which means 12 - 1 = 11V must be dropped across the bjt. 1V/8ohms = 0.125 A ----> power disipation of bjt = 11V*0.125A = 1.375W > 0.6 W max of the transistor. Is this correct?
5  Using Arduino / Audio / Re: Is this an appropriate circuit to drive my 8 ohm speaker using PWM? on: March 03, 2011, 02:36:18 pm
Okay I built the circuit, and the wonderful smell of magic smoke started to come out both bjts after decreasing resistance in the pot to 0 (I had to replace both). By the way, I kept the 4k and 1k voltage divider in the circuit. How do I calculate the power dissaption of the bjts in this type of circuit?
6  Using Arduino / Audio / Re: Is this an appropriate circuit to drive my 8 ohm speaker using PWM? on: February 28, 2011, 09:43:20 pm
Okay after taking your recommendations, I have adjusted the circuit to resemble this:



The diodes are to provide the dc bias to prevent crossover distortion, the pot for volume control, and 22pF cap for active low pass filter. What do you guys think?
7  Using Arduino / Audio / Re: Is this an appropriate circuit to drive my 8 ohm speaker using PWM? on: February 28, 2011, 08:35:57 pm
If I add active low pass filter would that cause my output wave to be triangular?
8  Using Arduino / Audio / Re: Is this an appropriate circuit to drive my 8 ohm speaker using PWM? on: February 28, 2011, 07:40:03 pm
I understand why there is distortion if there is no opamp there, but not when there is an opamp with the output is connected back to inverting input of the opamp. The output would always equal to the input * gain no matter if the transistors off when input is -0.7 -- +0.7V? Is this correct?

Edit: I did a Fourier analysis at the output with 10khz sine wave, and the THD is only 1.3%.
9  Using Arduino / Audio / Re: Is this an appropriate circuit to drive my 8 ohm speaker using PWM? on: February 28, 2011, 06:53:28 pm
Would feedback cause the output to compensate for deadzone problem?
10  Using Arduino / Audio / Is this an appropriate circuit to drive my 8 ohm speaker using PWM? on: February 28, 2011, 04:15:00 pm
Hi,

I was wondering if the circuit below will be able to drive a 0.2 W speaker?



The capacitor is used to remove the DC offset from the PWM output, and the voltage divider is used to reduce the vpp to 1 V. The 10 kOhm resistor is to reduce current to the OPAMP, and 20 kOHM is to multply the input by 2 so that the output wil be 2 V peak to peak. The transistors in the feedback loop helps to supply the required 125 mA to the speaker.

I don't have an scope to test this out, so I'm wondering if someone can verify that this circuit will not blow up.

Thanks,
s3n4te


11  Using Arduino / Motors, Mechanics, and Power / Re: Is this stepper unipolar or bipolar? on: February 05, 2011, 06:03:43 pm
Common connection for all the coils?
12  Using Arduino / Motors, Mechanics, and Power / Is this stepper unipolar or bipolar? on: February 04, 2011, 08:55:48 pm
The motor I'm using is the M35SP-9: http://www.mitsumi.co.jp/latest/Catalog/pdf/motor_m35sp_9_e.pdf

On the data sheet it says 2-2 phase unipolar excitation, but it only has 4 wires. Is this bipolar or unipolar?
13  Using Arduino / General Electronics / Re: Help reading current gain from BJT datasheet on: February 03, 2011, 05:48:18 pm
Also how would I figure out VCEsat? For example, I'm running Ib at around 14 mA, but only wanting to run 250mA from collector to emitter, how would I figure out VCEsat? I think I should be using Figure 16 on pg 6, but that doesn't give an graph for Ic at 250 mA. Figure 17 has VCEsat curve but that's for hfe = 10.
14  Using Arduino / General Electronics / Re: Help reading current gain from BJT datasheet on: February 03, 2011, 05:29:46 pm
Oops my bad! Okay, if I want a max of 600 mA, I should make sure that Ib >= 600/43 = 14 mA?
15  Using Arduino / General Electronics / Re: Help reading current gain from BJT datasheet on: February 03, 2011, 05:18:33 pm
So if I want to design for a maximum Ic of 5 A, I should make sure that Ib >= 5/120 = 41 mA?
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