Circuit: DIY Seat Belt Reminder
February 13, 2026
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While going through some back issues from the 1970s, I recently came across an interesting Elektor project from 1976. The Elektor seat belt reminder circuit was designed for the era when seat belt safety was only beginning to become a serious concern. Let's take a look.
The project featured two classic CMOS logic ICs: CD4011 quad NAND gates. They were used efficiently. First, as a latch (flip-flop) to “remember” that a door had been opened, and then as an oscillator to generate a flashing signal. Basically, opening the door armed the circuit, and once the door closed, the circuit would start blinking a warning light until it was reset.
"The circuit senses the fact that someone has entered either the driver or the passenger door by making use of the interior courtesy light door switches," Elektor noted at the time. "These are normally connected in parallel, but for this purpose they must be isolated by diodes. The interior light will then still function normally, but it is possible to sense the opening of a door when the other is already open."
Each seat had its own logic path. The driver side could be reset using a button once the belt was fastened, while the passenger side included an override switch, so the passenger warning could be disabled if nobody was seated. The lamps themselves were driven via transistor stages, so the CMOS logic didn't have to do the heavy lifting.
The design serves as a great example of how Elektor community members were already adding "modern" safety features to older vehicles decades before cars became rolling computers.
Editor's Note: This article originally appeared in a 1976 issue of Elektor. Due to the project’s age, some components might not be available. Still, we think the design will inspire you to start a new project of your own.
The Design
The goal was to develop a practical seat belt reminder for drivers and passengers. If someone opened a door, got in, and then drove off without fastening their belt, the circuit would flash warning light.The project featured two classic CMOS logic ICs: CD4011 quad NAND gates. They were used efficiently. First, as a latch (flip-flop) to “remember” that a door had been opened, and then as an oscillator to generate a flashing signal. Basically, opening the door armed the circuit, and once the door closed, the circuit would start blinking a warning light until it was reset.
"The circuit senses the fact that someone has entered either the driver or the passenger door by making use of the interior courtesy light door switches," Elektor noted at the time. "These are normally connected in parallel, but for this purpose they must be isolated by diodes. The interior light will then still function normally, but it is possible to sense the opening of a door when the other is already open."
Each seat had its own logic path. The driver side could be reset using a button once the belt was fastened, while the passenger side included an override switch, so the passenger warning could be disabled if nobody was seated. The lamps themselves were driven via transistor stages, so the CMOS logic didn't have to do the heavy lifting.
The Seat Belt Reminder Project
The original article, “Reat Belt Reminder,” appeared in Elektor July/August 1976. Check out the article.Editor's Note: This article originally appeared in a 1976 issue of Elektor. Due to the project’s age, some components might not be available. Still, we think the design will inspire you to start a new project of your own.
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