Having perused various threads on modifying espresso machines for improved shot quality, it seemed obvious to me that minimising temperature variation while pulling shots was a Good Thing.
I considered gronking but the cost didnt seem that much less than a full PID setup, given how strong the Aussie dollar is at the moment and the availability of cheap chinese PIDs on flea bay. So, I set myself a challenge to PID my ancient Gaggia for under $50.

Ill not rehash the step by step guides that are already out there on the internet, but thought I should share how easy and cost effective this mod can be. My main reference for the setup was one by "The Domestic Barista", but I also referred to several others.

The shopping list:

PID - I chose a Sestos D1S-VR-200 for AU$32.50
K type thermocouple - came with the PID
SSR - a cheap Chinese 25A unit for $AU6.25
Power cable for PID and AC to the SSR (free, came from an old sandwich toaster - silicone insulated to cope with heat)
Cable ties (free, had plenty lying around
DC cable for relay control (a short section of speaker cable is fine)
Crimp connectors - free as I already had plenty.
Double sided sticky tape - already had some

Total outlay $38.75!!!

My machine is a Silicon Graphics Espressigo (a Gaggia classic mounted in a computer case as a marketing gimmick), so had space inside the outer case to mount the PID unit. Most people will need to purchase a suitable equipment case. Dick Smith sell cases that fit for $3.98

Having got all the items above, next step was to monitor current temp cycles so I knew what was happening in the standard setup.

I hooked up power (using existing terminal block inside the rear of machine) and thermocouple to the PID, installed the thermocouple on top of the boiler underneath the thermal fuse as there is a convenient recess there that fits the thermocouple tip nicely. One warning - as the thermal fuse is no longer in direct contact with the boiler, there is a potential safety issue here if the PID or stem thermostat fails and overheats the boiler.
I wired up the SSR to the PID and attached short lengths of AC cables with crimp connectors so it can replace the stock thermostat. The SSR was then mounted on the rear of the case in direct contact with the metalwork - no need for heatsink.

Quick check of electrical connections and power up to review thermostat settings: heat up to 105C, ready light on temp rises to 113C, then falls slowly to 99C when heater kicks in, drops to 98.6 then rises again to 105C and cycle repeats. These temps are not necessarily exact brew temp as the stock thermostat is at the bottom of the boiler and my sensor was at the top (presumably hottest).

I then disconnected the two connectors from the lower thermostat and clicked into place on the crimp connectors from the SSR. On my machine, the existing spade connectors are right angle uninsulated items - plumbers silicone sealant seems to be a suitable insulating material as it can cope with heat and is not in direct contact with the brewing water/coffee.

A final check of all electrical connections and powered up again. Temperature set to 94.5C, it warmed up quickly, but varied from 95C to 105C. Ran auto tune and it took several minutes, then kept temperature between 94.4 and 94.6. Truly awesome, given how much lag there is between applying/removing heat and temp stabilising on the old thermostat.

I made my daughter a flat white (I dont use milk) - engaged steam switch, purged wand approx 10-15C and started steaming 6-8C before stock thermostat trips, AMAZING difference to quality of microfoam, no drop off in steam generation (previously a huge problem with the Gaggia).

Given how easy and cheap this mod is, Id recommend it to anyone who is electrically capable or has a friendly sparky to carry out the install