Using and output to power an 08M2

matchbox

Senior Member
#1
Over the last year, I have increasingly found the need to run parallel picaxe's.
In this current project. I am using a Hall sensor, like a rotary encoder.
The problem I am having here, is the same as I often have. Not enough processor time to run everything else and count pulses from the hall as well.

So what I am think of doing. Is to use an 08m2; just to do the counting of the pulses from the hall sensor. Then transfer the data to the main processor when it has the time to collect it.
But this project is power usage sensitive. And I don't want the 08m2 powered continuously. And the cable run is 1m between both picaxe's. So i would prefer not to have to run another lead to tell the 08m2 to turn off.
Instead, use the an output from the main picaxe to power the 08m2. The current drain of the 08m2 and the hall sensor will be lucky to make 10mA peak.
I know its an odd way of going about this. Does anyone see a problem with this?
 
#2
You should be able to make that work. The 08M2 output pin, being a relatively high impedance power source, would mean that you need both a 100nF and (say) 100uF electrolytic capacitors mounted at the remote PICAXE to keeps its power rail stable. As always, the 100nF should be mounted as close as possible to the PICAXE's power pins.
 

hippy

Technical Support
Staff member
#4
Powering from a PICAXE pin should work providing the current draw is within its limits.

An alternative is to pass a raw power rail to the target board and use a regulator with an enable input which can be controlled from the PICAXE.
 
#7
Here is a further option for higher current requirements. However, if your load is truly only 10mA, a simple output pin would suffice.

Hope this helps.

High current Pch FET switch.png
 

hippy

Technical Support
Staff member
#8
Here is a further option for higher current requirements.
It would probably be advisable to include a pull-up resistor on the FET gate so the slave PICAXE is kept off when the master PICAXE is removed from the circuit, is being reset or downloaded to.
 
#10
It would probably be advisable to include a pull-up resistor on the FET gate so the slave PICAXE is kept off when the master PICAXE is removed from the circuit, is being reset or downloaded to.

This is true and was omitted from the circuit provided due to the use of the controlling 08M2. in this case, a 1K resistor from the Pch FET GATE to 5V rail will do this.
 
#11
What logic-level P-CH fets would be recommended--through-hole and SMD?
This is a very broad question. The selection of FET would primarily be determined by the amount of current required at the switched output. Datasheets will provide this answer. Depending on the current, you may not even need a 'Logic Level' FET. When viewing datasheets, notice the graphs provided that show examples of VGS voltages vs DRAIN current. As an example, in the past I have used FDN340P @ 5V/1.5A continuous with a simple PICAXE output to switch it.
 
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