Lemon DIY module vs JR-style cased module

Like B&Q for homebrew radios
Post Reply
User avatar
Phil_G
Posts: 597
Joined: 15 Feb 2018, 23:32
Contact:

Lemon DIY module vs JR-style cased module

Post by Phil_G »

Following recent email exchanges some info has come to light directly from Frederick at Lemon, the DIY module is obviously intended for use in legacy transmitters and is very tolerant of PPM timing variations - in sync time, fixed sync or fixed frame, PPM pulse width, polarity, number of channels etc, it accommodates all equally well. Its just as happy on a 40-year-old Fleet or Skyleader as a modern Futaba.

The JR module is intended for the Taranis etc and uses an entirely different method of reading PPM, coded quite tightly to the JR spec.
It wont even begin to transmit until it sees a valid JR-compliant PPM signal. This strict requirement makes no sense to me, no-one else does it with JR-style modules - not even Spekky - but it does mean that if you are doing a legacy conversion, its best to use the DIY if you are not 100% sure the PPM uses precise JR timing.

The second point was accidentally discovered by Stu - the STM32 port used for PPM input has only a very weak pull-up to 3v3 and doesnt behave like Frsky, Corona, Hitec etc. Stu wanted to update a transmitter by swapping the old Corona module for a Lemon - and the set stopped working, because the Corona install had the inline schottky diode (low from the encoder pulls the PPM line lo a logic low, high 'releases' the PM line allowing the pull-up to take the input to a logic 1. WIth no strong pull-up, the Lemon didnt see any PPM. Remove the schottky (or swap it for a 1k resistor) and all is well.
Similarly Mike has done a straight Futaba conversion recently (ie using the original encoder), again no RF output from the Lemon until he added an external pull-up resistor to the PPM line (10k I think he said?). This isnt a problem with any of our homebrew encoders since they all output full TTL levels high and low - either Lemon module is perfectly happy with that.

The Lemons are great modules but I would advise using the DIY variant except in a genuine JR or Taranis etc.

Cheers
Phil
Post Reply