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Turbo46
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Posted: 20 December 2018 at 4:22pm | IP Logged Quote Turbo46

I feel your frustration.

I still think Rob's suggestion is close to the mark. There does seem to be some 'motor boating' going on. It looks like the supply is dipping to nearly 2 volts if I am reading your trace correctly. That may be causing the DP5 output to drop too low and turn the transistor off and therefore the FET thus reducing the load on the power supply allowing the voltage rise again and so allowing the DP5 output to rise again. This dipping of the supply is probably causing the processor to eventually lock up. If your circuit is correct and checked with a multimeter I would try more capacitance on the supply. Maybe heavier wiring to the supply?

Bill


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Turbo46
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Posted: 21 December 2018 at 4:15pm | IP Logged Quote Turbo46

Hi Timbergetter,

Please have a look here to see what Grogster says about the HC12 and power supplies.

Bill
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Timbergetter
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Posted: 21 December 2018 at 4:45pm | IP Logged Quote Timbergetter

I decided against stripping down the PCB looking for elusive impedances. Instead I have tried to implement the consensus proposals of Bill, Jim and Phil. So far it appears to have totally succeeded. This is the trace I am now getting:



The trace is for a supply voltage of 3.5 volts. The pulses start to appear for a lower supply voltage and / or for values of C7 below 220 µF. I am now solidly in the domain of battery operation (3 x AA alkalines and a sleep current of less than 6 micro amps). The schematic around the MosFet switch now looks like this:



Moving C7 away from the drain side of the MosFet seems to have had the biggest effect. R8 is probably redundant now, but I’ve left it in. The device appears to be sufficiently robust now to deploy and to focus on the receiver end of the system. I am so grateful for you guys who have helped me move this project along.
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Turbo46
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Posted: 21 December 2018 at 5:01pm | IP Logged Quote Turbo46

Great news! Thanks for sharing - we all learn from other peoples problems.

Grogster said in another post:
Quote:
Any kind of voltage-drop on the 3v3 rail will cause the HC12 to get confused and do odd things due probably to the brown-out effect.

Maybe that caused it to draw extra current and make things worse.

Bill
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CaptainBoing
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Posted: 21 December 2018 at 6:37pm | IP Logged Quote CaptainBoing

great thread.

I still wanna know why that is not a clean HI... if you stretch the pulse on D5, does the "ringing" continue (i.e. is it a feature of that hi)? with everything disconnected from D5?

bugging me...

Edited by CaptainBoing on 21 December 2018 at 9:33pm
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Timbergetter
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Posted: 28 December 2018 at 7:26pm | IP Logged Quote Timbergetter

Hello Captain. I am sorry I didn’t notice the edit to your post till now. You are wondering what is responsible for the pulses observed while D5 is ostensibly held high. During each cycle of D5 high I am writing 27 packets out to the HC-12 radio. For the purpose of illustration I have temporarily inserted a 500 ms delay after each packet sent. I think this makes it clear that these packets are the culprits causing the observed effect on D5, as seen on the attached traces. The mpu and the HC-12 are both supplied directly from the battery so I am guessing that what we see is the voltage drop in the battery’s internal resistance each time the radio is used. The pulses I was getting previously, and which seemed to be leading to instability, were distinguished by ranging between 0 volts and supply voltage.


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