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991  Using Arduino / General Electronics / Re: Differential line drivers for 100m twisted pair? on: June 20, 2011, 09:53:37 am
Code:
It seems strange that RS485/422 chips are the only things commonly available.

Why should it seem strange when that's exactly what you're trying to replicate
992  Using Arduino / General Electronics / Re: Weather proofing electronics on: June 20, 2011, 09:49:12 am
a vacuum cleaner may be good enough to get the air bubbles out.  Also warming epoxy type fillers with a hair drier reduces their viscosity.
993  Using Arduino / Project Guidance / Re: Super simple RPM monitor. on: June 19, 2011, 05:49:14 am
Glue some magnets to your drive wheels and fix a reed switch to the chassis.  As each magnet passes the reed a pulse is generated.  Cheap and simple.  One magnet will give one pulse per rev and 4 magnets will give 4 pulses etc

Then you simply feed the left and right pulses into a couple of dig-ins and compare the frequencies.  It matters not if the pulses are out of sync since it's frequency you're interested in, not phase difference.

Of course the higher the pulse rate the better the resolution, so if your motors are geared down you might be better looking at motor shaft speeds rather than drive wheels. 
994  Using Arduino / Project Guidance / Re: Measuring Current using IC`s on: June 17, 2011, 02:03:54 pm
use a hall sensor which is capable of measuring DC current - and gives a 0-5volt output over the measured range.  These are completely isolated from the measured circuit and place no burden upon it.
995  Using Arduino / General Electronics / Re: Drop 13.28v to 12v on: June 17, 2011, 05:31:47 am
Your alarm system mains power unit will provide very little power, probably 0.25 amp at most.  Your specified loads are voltage tolerant and would be perfectly happy running at 13.28.  Consider that a sealed lead acid battery (SLA) can run as high as 14 volts without crippling the devices connected to it.

You might be better getting hold of an old laptop power unit that gives out around 17 volts or so and feed that through a regulator chip to get your 12 volts.

Or alternatively keep your old alarm system as a power module, complete with battery, and power from it.  That way you can tolerate short term mains failures.  No good having a mains powered smoke alarm or LED lighting if the mains has failed and you have no back-up battery.  A bit like having a solar powered torch that only works when the sun shines.
996  Using Arduino / General Electronics / Re: Voltage Boosting Circuitry on: June 16, 2011, 01:42:00 pm
Get hold of a "disposable, single use" cheapo camera with inbuilt flash and take it apart to recover the inverter unit.  This will generate around your 400 volts from a AA battery.   If you talk nicely to your local photoshop they may have a stock of these in their rubbish bin.
997  Using Arduino / General Electronics / Re: magnetic reed switches - understanding ratings on: June 15, 2011, 01:58:16 pm
The 500ma refers to the MAXIMUM current that the switch can carry.   Since a reed switch is a sealed device it will happily switch extremely small currents (several microamps).   It's generally considered that many switches need a decent current flow so that the arc of breaking "wipes" the contacts clean, but this limitation doesn't apply to reeds.
998  Using Arduino / Project Guidance / Re: Advice on signal inverting circuit on: June 15, 2011, 07:30:22 am
or use an optocoupler for total isolation and do the inversion in software
999  Using Arduino / General Electronics / Re: How would I know if I short circuited anything on: June 14, 2011, 03:13:19 am
1) put power on and watch for the magic smoke  or

2) study EVERY wire, where it comes from, where it goes and compare this to the circuit diagram

There is no short cut to verifying a circuit is wired correctly.  It's either a case of checking and double checking or taking a gamble and switching on the power and accepting the consequences of any errors made
1000  Using Arduino / General Electronics / Re: Where to find precision R2R ladder? on: June 11, 2011, 03:22:42 pm
Assuming you want to power your control from the same supply as the telescope motoring device, and you want to read any combination of 6 switches then you will need to effectively create a 6 bit binary word where each bit (0 or 1) is an individual switch.  These you feed into a 6 input (binary) logic device which will read the respective binary word and produce a single analogue value.  As such you will need 3 wires for your hand control, 2 to supply power to the switches, DAC etc and one to return the respective analogue value.

However, 8 core security alarm cable is both flexible and of very small diameter so you might be trying to overcomplicating your requirements, when you could very neatly feed all 6 switches from your hand control back to the telescope.

1001  Using Arduino / General Electronics / Re: Correct capacitors for Voltage Regulator ? on: June 10, 2011, 02:17:50 am
Yes they will work but VASTLY overpriced and over-rated for what you need.  These are rated at 850volts DC.  All you need are something rated at 50 volts maximum - you'll find 100 volts or so pretty standard and cost less than £1 each. Something like this, though you'll find them much cheaper at the likes of Maplins
http://cpc.farnell.com/vishay-bc-components/2222-368-25334/capacitor-330nf-100v/dp/CA00694 

jack
1002  Using Arduino / Project Guidance / Re: Current measurement from Solar Panels on: June 09, 2011, 04:49:20 pm
DC hall current sensor is what's required with 0-5 volts DC output such as a CSLA2C or similarfor direct feeding into the arduino.
1003  Using Arduino / Project Guidance / Re: A variety of different frequency motors on: June 09, 2011, 07:51:31 am
This is really bouncing concepts off the wall. 
How about stripping out a musical box to salvage the reed assembly.  These are made of high tensile steel and are therefore magnetic.  Now, if you place a wound ferro magnetic coil adjacent to the reeds and excite this at various frequencies, only the reed which matches the excitation frequency will vibrate.
jack
1004  Using Arduino / Project Guidance / Re: variable frequency motors on: June 08, 2011, 04:29:22 pm
The only problem with changing motor speed is there is also a rapid change in amplitude, ie at lower speed there will be a gross reduction in amplitude and at higher speed a gross increase since the vibration is induced by an out-of-balance spinning mass.

One alternative is to use the natural frequency of a vibrating strip of metal.  Differing frequency of vibration gives you the required haptic feedback.  Use a coil to excite the strip.  Alter the natural frequency by driving a damper finger along the strip by use of a small servo motor.
1005  Using Arduino / General Electronics / Re: Battery Requirement Calculations - First Arduino Project on: June 08, 2011, 05:19:11 am
+5 on supply (battery or whatever) to ground on supply is OK for measuring voltage  -  but NOT current.  To measure current you need to connect meter +ve lead to +5 on supply and meter -ve lead to the +ve terminal of your device (arduino or whatever).  The device -ve lead connects to the supply ground
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