No regulator

[brownt]

what happens if I was to run a project on batteries without a regulator.

A microcontroller, a digital pot and a few LEDs

What sort of problems occur, or what is the disadvantage?

[MT]

what happens if I was to run a project on batteries without a regulator.
A microcontroller, a digital pot and a few LEDs
What sort of problems occur, or what is the disadvantage?

The "brown out" "Power Supply Supervisor "circuitry inside MCU will keep MCU at reset or reset or shut down MCU.
You basically dont do MCU design without regulator.

[james_s]

The disadvantage is you burn out the micrcontroller if your battery voltage is higher than the max rating, or the circuit malfunctions once the batteries are only partially drained and along with erratic behavior you end up throwing out batteries that still have a lot of life left in them.

[Ian.M]

Its not impossible if all parts have a sufficient supply voltage range.  e.g. Microchip have many 8 bit MCUs that  have a supply voltage range extending from about 2V to 5.5V which are well suited to running from 3x AA or AAA cells or a single LiPO,  as the supply voltage range includes both the fully charged voltage (and the max charging voltage for the LiPO) and the commonly accepted fully discharged voltage of 2.4V (0.8V per cell) for the 3x AA or AAA batteries, and 3.0V for the LiPO.

However driving LEDs without excessive brightness change as the battery discharges can be difficult, and its also more difficult to use analog sensors etc. when you don't have a fixed supply voltage.

One viable strategy is to use a high efficiency boost regulator IC to boost to 5.0V, when running, avoiding the common variable supply voltage issues, and shut it down in sleep mode so the MCU  gets Vbatt less one Schottky diode drop to maintain its state, + gate power to other parts of the circuit that don't need to be powered in sleep using P-MOSFETs.

[james_s]

Battery powered gear with no regulator used to be very common back in the days of CMOS logic. The widely used TMS1000 and related microcontrollers typically ran directly off a 9V battery. Not much advantage anymore though, efficient switching regulators are cheap and compact.