What Is an Open Loop System? Control Theory for DIY Electronics

The Core Concept: What Is an Open Loop System?

An open loop system is a control setup where the output has absolutely no effect on the input or control action, meaning the system blindly executes a command without checking if it actually succeeded. Think of it like driving a car blindfolded: you turn the steering wheel 45 degrees and hold it for five seconds, assuming you made a perfect right turn, but you have no way to verify if you actually stayed in your lane or drove into a ditch.

In electronics and embedded programming, this means your microcontroller sends a signal—like a PWM duty cycle or a step pulse—and assumes the physical world obeyed. There is no sensor, no encoder, and no feedback pin reporting back to the controller. What this changes in a real circuit is your Bill of Materials (BOM) and wiring complexity: open loop designs eliminate the need for feedback sensors (like thermistors, quadrature encoders, or current shunts), saving money and GPIO pins, but sacrificing precision under variable loads.

Component Spec Sheet: Open Loop vs Closed Loop Hardware

To understand how this theory maps to actual parts on your workbench, look at how manufacturers handle the same basic function with and without feedback. The table below compares common open loop components against their closed loop counterparts, highlighting the real-world trade-offs in cost, pin count, and performance.

Notice the cost multiplier. Moving from an open loop architecture to a closed loop system often requires adding physical sensors, more complex silicon (like the Trinamic StealthChop drivers), and significantly more firmware overhead to process the feedback data.

Worked Example: The Hidden Cost of No Feedback

Let’s put some real numbers to this concept using a classic DIY scenario: controlling the speed of a 12V brushed DC motor using an Arduino and an L298N motor driver.

The Setup:

• Motor Specs: 12V nominal, 3000 RPM no-load speed, 2.5A stall current, 0.2A no-load current.
• Winding Resistance ($R_w$): Calculated via Ohm's Law at stall (where back-EMF is zero): $12V / 2.5A = 4.8\Omega$.
• Driver Dropout: The L298N uses bipolar junction transistors (BJTs), which drop about 2V across the H-bridge under load. Maximum voltage reaching the motor is 10V.

The Command:
You want the motor to run at half speed (1500 RPM). You write analogWrite(enPin, 127), outputting a 50% PWM duty cycle. The effective voltage reaching the motor is roughly 5V (accounting for the 10V max after the L298N dropout).

The Reality Under Load:
With no mechanical load, the motor spins at roughly 1450 RPM. But then, you attach this motor to a conveyor belt and drop a 2kg weight on it. The mechanical load forces the motor to slow down, which drops its internal back-EMF. With lower back-EMF, the motor draws more current to fight the load—spiking from 0.2A to 1.4A.

At 1.4A, the voltage drop across the L298N increases to nearly 3V, and the voltage dropped across the motor's internal winding resistance ($1.4A \times 4.8\Omega$) consumes another 6.7V. The actual voltage left to generate rotational force plummets. The motor speed drops to 820 RPM.

Because this is an open loop system, the Arduino is completely oblivious. It continues outputting exactly 127 on the PWM pin. The error between your expected speed (1500 RPM) and actual speed (820 RPM) is 680 RPM, and the system will never correct it. If you needed that conveyor belt to move at a precise rate for a filling machine, your open loop design just ruined your product yield. To fix it, you would need to close the loop by adding a quadrature encoder and writing a PID control algorithm to dynamically increase the PWM duty cycle when the encoder reports a speed drop.

Where You Meet Open Loop Systems in Practice

Despite their lack of precision, open loop systems are everywhere because they are cheap, predictable in stable environments, and computationally lightweight. You will frequently design or troubleshoot them in these scenarios:

1. Home Irrigation and Sprinkler Timers

A standard $40 digital sprinkler timer is a pure open loop system. It opens a 24VAC solenoid valve for exactly 15 minutes because that is what you programmed. It does not know if it is currently raining, nor does it measure soil moisture. It simply executes the time-based command. (Adding a soil moisture sensor to interrupt the circuit turns it into a closed loop system).

2. Basic LED Dimming

When you use a MOSFET to PWM a 12V LED strip from an ESP32, you are running open loop. You set the duty cycle to 40% to achieve a specific ambient brightness. However, as the LEDs heat up, their forward voltage changes and their luminous efficacy drops (thermal droop). The ESP32 doesn't know the room got dimmer; it just keeps sending the 40% signal. For human ambient lighting, this is perfectly acceptable. For a photographic lightbox requiring exact lux levels, it is unacceptable.

3. 3D Printer Z-Axis Stepper Motors

Most budget 3D printers use A4988 or DRV8825 drivers for the Z-axis. These are open loop. The mainboard sends step pulses to raise the bed by exactly 0.2mm per layer. It assumes the motor moved. If the nozzle crashes into a warped bed and the motor stalls (skips steps), the printer continues printing in mid-air, completely unaware that the physical Z-position no longer matches the digital Z-position. Modern high-end printers are moving to closed-loop stepper drivers or sensorless homing to detect these stalls.

4. Resistive Heating Elements (Toasters)

A mechanical toaster uses a bimetallic strip that physically bends as it heats up, eventually popping the latch after a set time. It does not measure the actual temperature of the bread. It relies on the assumption that $X$ seconds of radiant heat equals $Y$ level of toast. This works fine for standard sliced bread, but fails if you put a frozen bagel in the slot, which is why modern high-end toasters have shifted to closed-loop thermistor feedback.