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196  Using Arduino / Project Guidance / Re: Wake up arduino from RF message on: November 09, 2013, 05:45:08 pm
Its not possible to make 433 Mhz receivers with extremely low power consumption, as most of the power goes into running
the oscillators and the RF amplifier.
There are some tricks you can use to reduce the power consumption however, which are commonly used in battery operated
weather stations.
It relies on knowing when the transmitters are likley to transmit and then turning the power to the receiver on and waking up
the Arduino from sleep for a short time .
For example, suppose you want to transmit data every minute.
You turn the Arduino on and the 433 receiver and wait until you get a valid transmission.
You then put the Arduino to sleep but keep time by using a low power 32.768 watch crystal.
When a minute has elapsed, you wake the Arduino up and supply power to the receiver.
This technique works , if the data to be sent is small , and comes at known time intervals.
197  Using Arduino / Sensors / Re: WIND SENSOR / ANEMOMETER on: November 06, 2013, 06:42:19 pm
You will have to pull the anemometer apart to see what type of sensor pickup it uses, otherwise you dont know
whether the voltages coming from the anemometer are even compatible with the arduinos input.
A common technique thats used is a spinning magnet which induces a small voltage into a pickup coil.
This produces an AC output which has to be conditioned so that the Arduino can read it.
198  Using Arduino / General Electronics / Re: Rectification Circuit (receiver of wireless power transfer) on: November 04, 2013, 04:36:52 pm
How far apart are the coils?
Most inductive charging systems need very closely coupled coils spaced only a few mm apart.
Hard to know what needs to be changed without more information about the primary coil circuitry.
199  Using Arduino / Project Guidance / Re: Transfer of charge wirelessly via inductive coupling on: November 03, 2013, 03:20:30 am
Ive repaired a number of Braun electric toothbrushes which use a close coupled inductive loop in the base of the toothbrush to charge the internal AA battery.
The oscillator in the charging base runs at 50 Khz and consumes 1.3 watts.
The power delivered to the battery charging cct in the toothbrush is 25 mw.
The coils are extremely close , around 3 - 4 mm spacing.
1.3 watts in for 23 mw out isnt very good , but in an application like this , the efficieny doesnt matter.
Trying to scale the idea up though , is very hard .

200  Using Arduino / Motors, Mechanics, and Power / Re: Need help on powering micro servo motor with 3.7 Li-ion battery? on: November 02, 2013, 12:43:53 am
Whats the current draw of your flashlight?
The capacity of a Li battery and indeed most other batteries is stated for a discharge rate of 1/20C .
At higher discharge rates , you will get less capacity.
201  Using Arduino / Project Guidance / Re: Transfer of charge wirelessly via inductive coupling on: November 02, 2013, 12:34:28 am
What is your goal here?
How much power do you want to transfer?
What will be connected to the secondary coil.
Your questions are too vague to really comment on.
202  Using Arduino / General Electronics / Re: cell phone jammer using house Mains ground as antenna on: October 29, 2013, 08:21:17 pm
To answer the original question the answer is no .
To make any kind of antenna that is in any way efficient , the antenna has to be resonant with respect to the frequency of the
energy being fed into it.
Most Cell phones operate on frequencies from 800 Mhz upwards.
Theres no way of connecting a transmitter on these types of frequencies to any kind of random length long wire antenna and expecting the wire to radiate the energy.
203  Using Arduino / General Electronics / Re: Easiest way to monitor a small solar panel on: October 29, 2013, 08:15:14 pm
What precisely are you trying to measure?
The output power of a solar panel varies depending on a large number of variables.
Primarily the solar radiation intensity , which changes over the course of the day, and whatever the solar panel is connected too.
Fixed value resistors will provide some data , but it wont mean much , as the values have to be changed depending on the level of light intensity.
To accurately measure how much power the solar panel can deliver over a period of time , needs a MPPT (Maximum Peak Power Tracker) which is an electronic device that continuously adjusts the apparent load to match the solar panels peak power point , and a battery to absorb the energy.
You could use a resistor instead of the battery if you dont care what happens to the Solar panels output.
You can make a MPPT charger with an Arduino if you wish.
Heres an article on how to do so.
http://www.timnolan.com/?page=arduino-ppt-solar-charger
204  Using Arduino / General Electronics / Re: How does Radio control of RC cars works without any antennas and using a LED? on: October 26, 2013, 02:10:37 am
IR generally works for indoor control, and they will work outdoor at night,or on very cloudy days  when theres no sun.
Sunlight totally swamps the receivers and they dont work.
Range is pretty limited.
205  Using Arduino / Sensors / Re: I am having trouble with a DHT22 temperature sensor to work on Arduino Mini Pro on: October 20, 2013, 07:08:08 pm
You need a pullup resistor of between around 3.3K and 10K on the sensor data line.
Without one , it might work , but will be very unreliable.
206  Using Arduino / Project Guidance / Re: Solar Powered Water Pump with Back up Battery on: October 20, 2013, 12:36:50 am
The energy needed to pump water is determined by the flow rate , ie how many litres per minute , and the head, ie
how far vertically the water has to be pumped.
Unless these values are known , its impossible to determine how much power you will have to generate with the Solar Panel.
207  Using Arduino / Networking, Protocols, and Devices / Re: Questions about WLANS, Wireless Networks, and Wifi on: October 18, 2013, 05:40:40 pm
OFDM is a technique used by services like digital TV transmitters and also digital radio to transmit
hi bit rate data is a bandwidth limited channel.
Transmitting hi bit rate data is hard, as it gets affected by transmission anomalies like frequency selective fading
and multipath.
To overcome these problems , OFDM was developed which transmits hi bit rate data at a slow bit rate, which might sound impossible.
To do this , the hi bit rate data is broken down into lots of low bit rate channels, which are all stacked next to each other and the whole lot is transmitted together.
At the receiving end , all the low bit rate channels are demultiplexed and the original hi bit rate data is reconstructed.
This makes the transmission quite rugged , and impervious to frequency selective fading and multipath.
208  Using Arduino / Networking, Protocols, and Devices / Re: Questions about WLANS, Wireless Networks, and Wifi on: October 14, 2013, 02:52:11 am
Its a frequency stability problem.
At 2.4 Ghz , to maintain a 1 Khz channel , needs a frequency stability of 4 in 10^7, which whilst possible needs
extremely good oscillators.
Theres also little need for 1 Khz wide channels at 2.4 Ghz .
Generally, the higher frequencies are chosen ,so that faster bit rates can be achieved.
How fast depends on many variables, but the theoretical limit is set by the bandwidth and the signal to noise ratio.
http://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem
Shannons theorem, as its commonly called, explains the relationship between maximum data rate, channel bandwidth
and signal to noise ratio for communication over a noise limited communications channel.


209  Using Arduino / Project Guidance / Re: Outdoor & Indoor remote weather/temp system on: October 13, 2013, 04:41:39 pm
Its easy to build a temperature / humidity sensor , but a lot harder to build a rain guage.
Ive built a number of temperature / humidity sensors using the DHT22 , a small micro and a 433 Mhz transmitter.
Some issues you need to think about is do you want to expand the system later and add more sensors.
210  Using Arduino / Motors, Mechanics, and Power / Re: Using LiPo Fuel Gauge for reading THIS! battery pack on: October 12, 2013, 03:58:16 am
Dont confuse precision with resolution.
The Arduino can read to a resolution of 4.9 mv , but that doesnt mean its accurate to 4.9 mv.
Its accuracy is set by the accuracy of whatever its using as its voltage referance.
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