Before I upgrade my 4.0, should I buy a HM? What version HM is this one?


New member
Hi Guys,

Back after several years. Just fired up my 4.0 and had a great smoke of 2 shoulders at 235 and 4 racks of turbo ribs at 350 a week ago. I got the bug back for taking the next step with my HM. I would love to add a servo damper to better control overshoot & temperature. I had been talking with Ralph about buying a Roto Damper, and he's suggesting the 4.2 upgrade to my 4.0. I'm a bit apprehensive to undertake the blower circuit update, along with the noise filtering on the probes to do the update. I'd be OK to solder on 2 leads to a connector, but not that full surgery, and Ralph said he'd be concerned with the effectiveness of just soldering on the 2 leads without the other actions.

So I'm thinking about what to do next. Do I build a new 4.2 to replace my 4.0? Find someone to update my 4.0 for a friendly fee? Or buy a new one?

Looking for recommendations.

I saw someone on Ebay selling this kit:

I sent the seller a message. Trying to determine which version of the HM this is. 4.1? 4.2? And where this kit might have come from. Any ideas?

Thanks in advance.



TVWBB Diamond Member
To be clear, I am not concerned with the effectiveness of soldering a servo to the ATMega, it does work. The concern I raised was probe noise causing the servo to be overactive. The updates to the HM after v4.0 were made to address this issue.... Voltage mode for the blower is less noisy, R/C filters on the probes eliminates noise, and the servo booster circuit increases servo stability.

Bryan Mayland

TVWBB Honor Circle
Wow that was a really good deal for that HeaterMeter on eBay. A whole setup with probes for $120? It definitely looked like it was a 4.2.

I'm all about using what you've got and just making it better, and adding a servo is an easy mod if you've got a way to connect the wires. I think us veterans ran like that with a modded 4.0 for a while before we integrated it all into the standard board. As Ralph pointed out, there are a number of other improvements to the 4.2 boards apart from just the servo addition and I'd not really recommend anyone try to do the other changes as a mod to a 4.0 unless they're a real hardcore tinkerer. You've already got the Pi and power supply and blower so that's half the cost of a 4.2 system right there.

It's a real tossup, but you could definitely just solder Ethernet wires directly to your 4.0 and plug the other half into the RD3. The cost there is just the cost of the cable, and if you find that you're really digging it but it end up wanting the other features you can upgrade at that point and only be out a couple of bucks.


New member
Well that eBay listing sold faster than I could get my questions answered, so I missed out on it. It would have been an easy way to make the 4.2 upgrade happen.

I'll ponder my next step. I'm not sure if I add the Cat 5 and run the servo off my 4.0, or if I build a 4.2.

If there's anyone selling a soldered up working 4.2, I may just buy that and a case and move on.


TVWBB Diamond Member
That would have been one hell of a deal for all that HM hardware @ $120!
Mod'ing the blower circuit may be a bit daunting, although doable if you are patient and follow the pictures closely.....
Adding the R/C filters on the probes is pretty easy though, just cut the traces with a razor and install the resistors between the probe jacks and the ATmega, and add the capacitors across the probe jacks. Cheap and very easy IMHO, but yields a big improvement in probe stability...
Post #26 in the following thread shows details:
I used 1/4 watt through hole resistors because it was what I had on hand, however, the SMD resistor option would be much neater on v4.0 and should be pretty easy to pull off with a soldering iron and a pair of tweezers.
I would suggest you look at the R/C filter mod at least, big improvement for minor / inexpensive surgery, will make the servo run more smoothly.
The servo can connect right to the ATMega via whatever type connector you decide to use (need three wires for the servo plus 2 for the blower if you want them all on the same connector with v4.0).
If you have problems with servo jitter an inline servo booster is super easy to build, see my post here about it.
If you had a v4.1 board I would push you more to do the blower circuit update, as the v4.1 board had a lot of noise issues with the blower, v4.0 was a better board IMHO and had less blower noise issues. The major issue with v4.0 blower circuit is it pulsed the ground instead of the +!2v so it cannot share a ground with the servo, so it requires 5 wires to run the blower and servo rather than 4 like the modern circuit... but if you are doing a one-off wiring and can accommodate the extra wire that isn't a big problem. I would suggest it would be pretty easy to add the transistor and resistor that make the servo booster when you're making your custom wiring.
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