714 IEEE Open-silicon 2026 x NITHUB: Fluid Level Detector and Controller :: Quicker, easier and cheaper to make your own chip!

About this project

This design was created as part of the IEEE Division 1 Open Silicon Initiative in Lagos, Nigeria, headed by Emmanuel Innocent. The initiative trains participants in modern digital integrated-circuit design — from RTL and FSM modelling through to a hardened ASIC layout — and partners with Tiny Tapeout to provide a free silicon tapeout opportunity for completed projects.

Without the programme there would be no realistic path for a student in Lagos to get a custom design fabricated on real silicon; the combination of the Open Silicon Initiative's instruction and Tiny Tapeout's shared shuttle makes that possible. This Water Level Controller is one of the projects produced through the programme.

How it works

This design is a 4-state finite-state machine that automatically controls a water pump from two binary level sensors.

The two sensors are bundled into a 2-bit sensor_bus = {s1_high, s0_low}:

`sensor_bus`	Meaning
`00`	Tank empty (water below low sensor)
`01`	Mid-level (water above low, below high)
`11`	Tank full (water above both sensors)
`10`	Impossible (high wet, low dry) -> safety fault

The FSM has four states:

IDLE (00) - pump off, waiting. If the tank reads empty (00) it moves to FILLING. If the impossible pattern (10) appears it moves to ERROR.
FILLING (01) - pump on. Stays here while the tank is empty or mid-level. Goes to FULL when both sensors are wet, or to ERROR on the impossible pattern.
FULL (10) - pump off. Stays full until the tank drains back to empty (then re-enters FILLING) or the impossible pattern appears (ERROR).
ERROR (11) - pump off and error_flag raised. Stays here until both sensors clear to 00, then returns to IDLE.

reset_n is an active-low asynchronous reset that forces the FSM into IDLE.

How to test

Drive the two sensor inputs on ui_in[1:0] and watch the outputs on uo_out:

Pin	Direction	Function
`ui_in[0]`	in	`s0_low` - low-level sensor
`ui_in[1]`	in	`s1_high` - high-level sensor
`uo_out[0]`	out	`pump_out` (1 = pump ON)
`uo_out[1]`	out	`error_flag` (1 = impossible sensor combo)
`uo_out[3:2]`	out	Current FSM state (`00` IDLE, `01` FILLING, `10` FULL, `11` ERROR)

A typical test sequence:

Apply reset (rst_n = 0) for several clocks, then release. State should be 00 (IDLE), pump off.
Drive ui_in = 2'b00 (empty). After one clock, state -> 01 (FILLING) and pump_out = 1.
Drive ui_in = 2'b01 (mid). State stays at FILLING, pump still on.
Drive ui_in = 2'b11 (full). State -> 10 (FULL), pump off.
Drive ui_in = 2'b00 again. State -> FILLING, pump on (controller refills).
From any state, drive ui_in = 2'b10. State -> 11 (ERROR), error_flag = 1, pump off.
Drive ui_in = 2'b00. ERROR clears back toward IDLE / FILLING and error_flag drops.

The cocotb tests in test/test.py cover all of these transitions automatically.

External hardware

Two level sensors (float switches, capacitive probes, optical sensors, etc.) wired to ui_in[0] (low) and ui_in[1] (high). Pull-downs recommended so a disconnected sensor reads 0.
A pump driver / relay / MOSFET driven by uo_out[0]. The chip pin cannot drive a pump directly - use a transistor or relay module.
Optional: an LED on uo_out[1] to indicate the error state, and two LEDs on uo_out[3:2] to display the FSM state.

#	Input	Output
0	s0_low (low-level sensor)	pump_out
1	s1_high (high-level sensor)	error_flag
2		state[0]
3		state[1]
4
5
6
7

714 IEEE Open-silicon 2026 x NITHUB: Fluid Level Detector and Controller