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1066  Using Arduino / General Electronics / Re: Do I need to use a resistor for a small speaker? on: March 14, 2011, 02:04:32 pm
On the basis that the arduino can happily supply around 25ma at 5 volts, the output power is around 100mW.  This can be heard on a small speaker OK but certainly won't blow your ears.  The speaker circuit impedance should be around 200 ohms (5000/25).  Most small speakers are about 8 ohms so you are going to need a series resistor of around 192 ohms (let's say 180)  since the speaker will now only "see" 8/180 of the 200mW, ie around 8mW, you'll be lucky to hear anything.

Better to use an op-amp to act as an amplifier between the arduino and the speaker, as well as permitting the speaker to be connected directly to the amplifier (without a series resistor).

1067  Using Arduino / General Electronics / Re: measuring very low AC voltage on: March 14, 2011, 04:42:23 am
Clearly you need what is referred to as a "precision rectifier" circuit.  In these a simple opamp has a feedback loop which takes into account the normal forward voltage loss across the diodes.

Try googling "precision rectifier" or go directly to for a write-up on the subject

1068  Using Arduino / General Electronics / Re: Voltage divider 101 help, any suggestions? on: March 14, 2011, 04:33:11 am
What is your green wire marked as D- connected to.  One end of the 68k resistor appears to be connected to it but where does the other end of the wire actually go to.  This end needs to go to your ground reference point so that the 100k/68k connection point (blue wire) can develop the correct 2volts.

You also need to establish how you are measuring the 3 volts.  I presume your +ve lead is connected to the 100k/68k junction (blue) but where does your -ve lead connect (the other end of the 68k ?)

When doing breadboarding like this you should draw out your circuit and mark exactly what wire connects to where.  That way, when you build your breadboard you have a means of verifying that you have connected the components correctly.

By the way your 5 volt supply is not providing 500ma to the circuit.  When a power supply has a current rating specified it simply means it is capable of supplying that amount of current

And finally, if the circuit that you are connecting your 2 volts to draws any current then your 2 volts will drop since the external load is effectively in parallel with your 68k resistors.  You can minimise this effect by reducing the source resistance.  Simply change your resistor chains by a factor of 10 or even 100 so that you have 10k plus 6.8k  or even 1k plus 680ohms.  With the latter components the power dissipation across the resistors will be minimal (0.025watts at worst)  the loading effect of your external circuit will be negligible.

1069  Using Arduino / Motors, Mechanics, and Power / Re: Regulator shouldn't e heating up like this! on: March 12, 2011, 04:21:32 am
Have you actually measured the output of your wallwart with the arduino connected
If that measures OK (<12volts) measure the current being drawn by the arduino.

The heat developed by the regulator is (Vwallwart-5) x Iarduino watts
Anything more than around 0.2 will cause the regulator to get somewhat hot since there is no heatsink fitted

Voltage regulators are specified as being capable of quite high input voltages and high output currents but these only apply if adequate cooling facilities are provided.   Without a heat sink you can have reasonable voltage drop but only at very low currents.

If voltages and current are reasonable you could try adding a heatsink to the regulator

1070  Using Arduino / General Electronics / Re: cheap and simple audio delay schematic? on: March 10, 2011, 04:31:43 am
Now this is an interesting project.

However, you also need to take into consideration the mechanical limits of your vibrating bars.  When a bar "sings", if you then further excite it by feeding back a positive resonance signal, the bars amplitude of vibration will increase (the design intent) which in turn will give a greater output, which will increase feedback, which will increase output ------- ad infinitum.

So you need to, not only provide suitable phase shift to give positive feedback but also to limit the amplitude of excitation to prevent either end-stop bashing or self destruction, and develop a means of effectively controlling volume output.

1071  Using Arduino / Motors, Mechanics, and Power / Re: Transistors letting volage "leak" through? on: March 09, 2011, 11:48:46 am
Quote " yes battery is 9 volts, well ive been using my 9v wall wart "

Contradictory and/or confusing statements and misleading pictures makes it kind of difficult to establish exactly what you meant.

Never the less the comment about high starting current and collapsing supply voltages still stands.

Measuring across the terminals of a DC motor to establish resistance tends to give confusing information as the brush to commutator resistance is liable to change dependent upon numerous variables including commutator stop position, cleanliness of commutator, brush pressure etc.  The stall current is also dependent upon start friction (stiction), shaft load etc

Measure the wallwart output when you are trying to run your motor but before you help them and no doubt you will see where the problem lies. 

1072  Using Arduino / Motors, Mechanics, and Power / Re: Transistors letting volage "leak" through? on: March 09, 2011, 03:32:37 am
If you really are using a battery of the type your illustration shows, then it isn't surprising your motors do not turn unless encouraged to do so.  This type of battery can supply very little current and to start a cheap DC motor requires a fair amount of current.  The current demand falls off rapidly as the motor accelerates.  This high input current will also be contributory to why the transistors are overheating.
Use a "stronger" (larger) battery if you wish to continue on 9 volts or else get an sla type 12volt battery.  These permit serious experimentation and are rechargeable.
1073  Using Arduino / Motors, Mechanics, and Power / Re: connecting 12v water pump on: March 08, 2011, 03:56:52 am
No, something like a 1n4002 to 1n4007 series or similar for this application, rated at 100volts+ and 1 amp, since all it's doing is "quenching" the reverse voltage kick when you switch off the power.
1074  Using Arduino / General Electronics / Re: Nixie Tube Transformer on: March 08, 2011, 03:46:38 am
You need a couple of capacitors on its output, say 0.1uF and 100uF in parallel.  One takes out HF noise and the other LF noise.  Within the current capabilities of the chip, the output voltage should be reasonably independent of the current draw, so it's probably the oscillation of the output that is giving you the "1 volt"
1075  Using Arduino / Motors, Mechanics, and Power / Re: connecting 12v water pump on: March 07, 2011, 05:39:28 pm
Try it and see.  You will need a heat sink and a reverse connected diode across the motor terminals to protect the transistor when you stop the pump.
1076  Using Arduino / General Electronics / Re: current limiting between GND & 5V on: March 07, 2011, 05:31:25 pm
The only sort of volume pots that you might have trouble with are the type sometimes used to control volume by direct connection to the speaker, rather than the amplifier input stages.  The direct speaker pots are very low value, typically around 10 ohms and invariably wire wound.
First thing all experimenters should do is buy themselves a digital test meter.  Provided you are not looking for a professional model you can buy these for around £10/$15.  A basic meter permits you to measure resistance values as well as voltages and currents.
1077  Using Arduino / General Electronics / Re: Combining analog input on: March 07, 2011, 05:22:04 pm
You might also want to consider fitting decoupling capacitors (0.1uF) across Vcc and Gnd on each of the chips to prevent noise affecting operation of the signals.  These should be fitted as near the chips as possible.
1078  Using Arduino / Motors, Mechanics, and Power / Re: Increasing Wall Wart voltage? on: March 06, 2011, 04:35:50 am
I'd suggest you find another one.  However, the first thing I'd do is measure the actual output voltage. If it's a cheap one with a transformer you will no doubt find that its output is greater than the specified 5 volts at currents lower than the maximum load current.  Don't be surprised if you find it as high as 9 or 10 volts.  Cheap wallwarts are not renowned for producing quality output.
1079  Using Arduino / Motors, Mechanics, and Power / Re: What does the 500 Hertz pulse-width modulation mean? on: March 04, 2011, 06:24:24 pm
I think you are confused as to what the number range 0 to 255 represents.
The PWM frequency of 500HZ is the cycle frequency between logic values 0 and 1 that the arduino is outputing, where logic0=0volts and logic1=5volts.
This frequency is constant irrespective of what the duty cycle is.
Duty cycle may be considered as the percentage that the cycle is at logical value 1.  The arduino uses the number range 0 to 255 to represent the duty cycle where 0= no output, 127= 50% at 0 and 50% at 1, 255=100% at 1
Duty cycle may be calculated as   (numerical value*100%)/255
Trust this helps clarify
1080  Using Arduino / Motors, Mechanics, and Power / Re: Stepper Motor Knob on: March 02, 2011, 05:49:19 pm
On the photograph you have supplied it appears that the top connection on the pot isn't soldered to the connecting wire.  This might be the source of your erratic behaviour.
Apart from that I like your "breadboard" (or should that be MDF) layout
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