Radio Control Servo Pulsin Mystery

[dj_trippn]

Hi all,

I have a little robot project where I am reading a servo channel output from a futaba radio control receiver into a 28x1 using the pulsin command. I have this working on my test bench but as soon as I tried powering it from a battery supply it stops working.

I have a simple loop that reads the pulsin command and then displays this to the PE terminal. If I use my benchtop 5V supply to power my circuit it works fine (actually 4.94V). If I use the RC receiver battery pack to power the picaxe it works fine (4.6V). If I use a 11.1V li-po battery through a 7805 voltage regulator with caps the picaxe does not receive any pulses (5.07V).

This seems strange. The rx receiver is still definetly working because if I plug a servo into the channel I am reading it moves it accordingly. I am wondering if the different voltages are causing a problem. There shouldnt be an issue using the 11.1V li-po with voltage reg?

I obviously have the 0V from the rc receiver pack and my supply connected together. I have even tried powering the rc receiver from my regulated 5V and this made no improvement.

Any suggestions why the pulsin command would suddenly start reading zero when using this different voltage supply?

Thanks all

 

[BeanieBots]

You've done the obvious so this is strange.
It's worth double checking that nothing 'odd' happened when changing over to the 7805. In particular, check that the 0v's really are connected.

If both the receiver and PICAXE are powered at the same voltage, it should work

I don't suppose you have access to a 'scope?
If yes, what are the max/min voltages of the incomming servo signal?

 

[Buzby]

Hi,

As BeaniBots says, you seem to everything right.

Have you got a variable bench supply ?. If so, try that in place of your LiPo. Try at different voltages between 10-15v. See how it behaves.

When you say '7805 with caps', I presume you mean electrolytic and ceramic ?. 7805's can oscillate at one voltage, but be stable 0.2v away.

Put an activity led on the PICAXE, just toggling every second. If this doesn't flash then it's the PICAXE that's stopping, not just the pulsin.

Let us know how you get on.

Good luck.

 

[westaust55]

Buzby raises a good point.

78xx series voltage regulator datasheets generally (there will always be one that does not) specify that a ceramic, and possibly also a tantalum type, caps be fitted at the output to prevent the 78xx regulator going into oscillation.

Eg: Notes: (3) Co improves stability and transient response

Typically 0.1uF ceramic and a 10uF tantalum in parallel is a good starting point.

 

[dj_trippn]

No I don't have a scope, but I would love to see whats going on with the supply and rc reciever channels.

I am using a old computer power supply for my benchtop which has 3.3, 5 and 12V rails. After doing some more testing today I realise that it is not always working reliably on the power supply either. The only time it is working reliably is with the RC RX battery pack powering the picaxe.

I realised a flaw in my original design now, which is I am using two servos modified for continuous rotation but I cannot run these through the 7805 as they draw far to much current. I forgot about this but quickly realised when I was testing and had a servo connected to another channel on the rx (5V supplied from li-po through 7805) and the 7805 started smoking. I quickly unplugged and it was smoking hot. The funny thing about this was the axe started registering pulsin. Could this be due to the effiency or output from the 7805 changing as it got hot?

I didnt fry anything luckily. How noisey is a computer power supply 5V rail?

Another weird thing I noted was if I was powered from the computer power supply with the servo connected to the rx the picaxe wasnt reading any pulsin's but if I started moving the servo connected on the other channel the pulsin's started reading the correct value and then stopped once I stopped moving the servo around.

Anymore ideas?

Thanks heaps

P.S. I have a 100uF electrolytic and 0.1 tantalum in parallel on my supply. When should I be using a tantalum or ceramic? What is the difference? I think I will need to get a new supply of caps. As not a clean supply seems like the probable cause.

 

[BeanieBots]

I thought you were using a LiPo pack into the 7805? (first post)

Computer power power supplies are NOT suitable for PICAXE supplies.
There are many reasons for this.
First off, they are very high power and can do a lot of damage if wired wrong.
They are multi-output switchmode units. If the 5v line is not correctly loaded (often many amps), the other outputs will not work and the 5v could be unstable.
In addition to the above, the 5v will quite likely have high frequency noise on it.

Above all, the 12v line is only good for low current and will only work properly when the 5v line is loaded as intended by the 'normal' use of the supply.