RF TX Power control

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Hooter

Senior Member
Folks - A while back there was a discussion on powering up a wireless module after a Picaxe had been in sleep mode - for longer battery life. The TX would then do its thing and then the whole unit would sleep for a while, wake up and start the process over again.
I have searched for a number of hours for this article mainly for the component aspect - it involved a couple of transistors, or a FET - but alas cant find it.
Could someone please point me in the right direction.
Hooter
 
D

Dippy

Moderator
Depends on the RF module.
What voltage supply? What current?
Some have Enable.

Is it really anything more difficult more tricky than a P-Chan or PNP for a tiddler?
A real tiddly tidder could be powered from a pin.
 
M

moxhamj

New Member
There is a circuit using a BC547 which pulls the base of a BC557 down using an appropriate resistor (?2k). The current is determined by the high side transistor - 100mA for a 557.

I've been thinking through some implementations that don't use the 547 but I think there may be parasitic powering issues to consider if the picaxe goes to sleep too.

Do you need the complete circuit ie with notes on the way the emitter is different for 547 and 557 ie current comes out of an emitter on a 547 but goes into the emitter of a 557?

Or you can always go for low side switching and leave 5V always connected to the radio module and switch the ground on and off. Then you can do it with a single transistor or mosfet.

Which RF module and how much power does it use?
 
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lbenson

lbenson

Senior Member
Without having done direct comparisons, I would tend to say that you can do very well without sleeping and having external wake-up circuitry, but by slow-clocking down to 31kHz as in the first thread ec referred to: http://www.picaxeforum.co.uk/showthread.php?t=8353

This measured 31 microamps while looping waiting for input, and a version which takes a temperature or two every 5 minutes or so and does a 433mHz transmission when there is a change will last 3-4 months on 3 Duracell AAs.
 
S

SAborn

Senior Member
Some of the RF modules go into low power state if no data is presented for a short period anyway.
It is more important to use N2400 or what ever baud compared to T2400 as with N2400 the TX only goes high when sending data, but with T2400 it is always high and goes low when sending data.

I get stronger transmittion with T2400 but at the cost of greater power consumption.
In my case it is plug pac supplied so no problem but would be if battery supplied.

Use N2400 and you might not need all the fancy control gear. Worth testing out.
 
M

manuka

Senior Member
Hooter: One assumes you've considered aspects below? DisableBOD saves HEAPS more power in sleep mode! Andrew Hornblow reports 2µA idle from a "BOD'd" 08M,& they stay alive down to 1µA at 1.3 V.
Here is a summary, by PICAXE Forum poster Ibenson, based on work done by a number of people:

Low-power features-several techniques suggested in previous posts to reduce PICAXE power.

1. Run at slower speed. Poke the OSCCON register ("poke$8f,%00000000") to run at 31.250kHz instead of 4mHz (1/128th the speed).See "08M UNDER clocking"-- http://www.picaxeforum.co.uk/showthread.php?t=2233 ; also http://www.kranenborg.org/ee/picaxe/ (search for low-power) also search the forum for "OSCCON".

2. Run DISABLEBOD. This turns off a PICAXE brown-out detection, which normally causes an 08M to shut itself off at a bit below 3V. With DISABLEBOD, 08Ms have been kept alive down to 1.3V. See manuka, et. al.:http://www.picaxeforum.co.uk/showthread.php?t=6513 ; also Mycroft2152, "Low PowerPICAXE 08M - BOD" -- http://www.picaxeforum.co.uk/showthread.php?t=4826

3. If you are using occasional ADCs while on battery, turn off the ADC module between reads by poking the ADCON0 register ("poke $1f,value"). Bit0 of value turns it off, but the register should be read first so that other bits are preserved. Quote Dippy "So for low-power people (doing the occasional A/D) it looks like a quick poke might be in order to switch off ADC module" http://www.picaxeforum.co.uk/showthread.php?t=5950 ; also http://www.picaxeforum.co.uk/showthread.php?t=8346

4. Pull down all unused inputs to 0V, e.g. with 100K or even 1M resistors. See "problem with getting very low power consumption" http://www.picaxeforum.co.uk/showthread.php?t=2292

Quote (Kranenborg) "After I had tied all 18X inputs low current consumption dropped from a very unstable 250 uA to a very stable 30uA at 5V and 31kHz (equiv. current consumption at 4MHz is approx 1.5 mA = 1500uA thus meaning a reduction to only 2% of the original consumption)".

5.Current-limit any outputs to the degree possible. For example, at 4.5V, an LED with a normal 330 Ohm dropping resistor uses about 13 mA. With a 22k resistor it is much dimmer,but still readable, & uses only ~.2 mA. Newer ultrabright LEDs drawing such sub mA currents can often be viewed at some distance, & at ~1mA may be almost blinding. LED "blink patterns" can further reduce overall current usage while expanding the information provided.

NB: For any register poking, confirm via Microchip datasheets that you're poking the correct one for that chip!
Click to expand...
 
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H

Hooter

Senior Member
Thank you to all who have replied.
Yes I had read all of the excellent contributions from IBenson and others and will be using those suggestions.
The module I will be using is the Hope HM-T 2.5v to 5v at 30ma when transmitting - thus unable to use an output pin of the 08M.
http://www.hoperf.com/rf_fsk/HM-T.htm
(USD $3 each with USD $3 China Post and arrived in Australia 7 days from HopeRF)
It doesn't have an enable input and as they have only just arrived I haven't had time to test the quiescent / idle current yet.
My intention was to monitor a door reed - along with other devices - by having the transmitter disabled and the 08M in sleep mode. The 08M would come out of sleep mode, check input status and on change of status only then power up the transmitter, send a burst then shut the transmitter down again until required.
The 08m would also occasionally send an 'i'm still here' call to the receiver.
I recall there being something akin to this spoken about previously. Could have been a long time ago.
I will have a play with the high and low side switching as suggested by Dr Acula.
Thanks again to everybody.
 
lbenson

lbenson

Senior Member
In the cheap line (that's for me, for my usage), those Hope FSK modules look superior to the e-MadeinChn ASK modules I've been using, and for only $.50 more. I'll try some out. Thanks.
 
lbenson

lbenson

Senior Member
In a tight loop like the following, the 08M need never sleep and can monitor the door in real time, going to full speed to transmit.
Code: 
symbol ADCON0 = $1f   ' ADC control SFR register to turn off ADC module
symbol OSCCON = $8f   ' Oscillator control SFR register for speed up/down

    poke OSCCON,%00000000 ' = 31 kHz slow down
    for b1 = 1 to 240 ' idle for 6 minutes--1.5 second loops
      if pin1 = 1 then
        ' do something
        b1 = 240
      endif
      pause 8   ' ~1.5sec or 6 min @ 240 loops (@ 31KHz)
    next b1
    poke OSCCON,%01100000 ' = 4 MHz; speed back up
Note that the timing is not very accurate, and varies with temperature.

Code is just meant to be suggestive.
 
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M

manuka

Senior Member
I've been keeping an eye on diverse 433 MHz offerings ( with Dr_A & I particularly taken with a new SURE GFSK transceiver), & agree these may have appeal- especially given the flexible supply. Have you noticed the transmitter power can be wound back? As the full power range is ~200m LOS, such a tweak may be enough to reduce supply current to ~20-25mA - a level that allows direct PICAXE control. What's wrong with slipping in a simple NPN "buffer" however?
 
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