Short answer
Buy an uncharged Hypno-device from Gunter, the astrologer, once the port is open. Charge it at the Formulation Bench in the Abandoned Tower with 1 Hypno-device and 1 Faith (mastery 2). Charged devices go into the PR Machine on Gluttony day, where they raise your zombie limit for the next few weeks.
The Astrologer's Tower
View full-size image ↗Official promotional screenshot · © Lazy Bear Games / tinyBuild · Official media kit · Material credits
Where to buy one
Hypno-devices are sold by Gunter. You meet him after progressing the main story far enough to unlock the port area.
Fextralife places his shop in the Astrologer's Tower.
What he sells is the uncharged device — it has no use until you charge it.
How to charge it
Take the device to the Formulation Bench, which sits in the Abandoned Tower on your base.
The recipe is 1 Hypno-device plus 1 Faith for 1 Charged hypno-device, and it needs mastery 2. The Alchemist perk can add a bonus device.
If the bench is missing, research Church Lighting I — that is the technology that unlocks the Formulation Bench.
Faith comes from church ceremonies, so charging a batch is easiest right after a good weekly sermon.
The Congregation Trouble quest
The first charged device is a quest item. Return it to Gunter and hold a test sermon to progress Congregation Trouble.
Completing the quest unlocks the PR Machine and gives 10 Gunter reputation.
From then on, every further charged device is fuel for that machine.
What the PR Machine does with it
The PR Machine raises your zombie limit so you avoid the Excessive zombies debuff on sermon income.
Load it on the Gluttony-symbol day. The +10, +20 and +30 programs take 2, 3 and 5 Charged hypno-devices; bigger programs add Newspapers.
The bonus lasts only while a run is going, which is why you will keep buying and charging devices for the rest of the game.
The graveyard setting from the official media kit. It is not a PR Machine or Hypno-device screen.
View full-size image ↗Official promotional screenshot · © Lazy Bear Games / tinyBuild · Official media kit · Material credits
A weekly routine that works
After Mass, spend a little Faith charging the devices you will need for the next PR Machine run.
Keep a few uncharged devices in stock so a Gluttony day never catches you empty-handed.
Check the zombie count against the limit before you raise new workers — our zombie limit page explains the numbers.
