kair.us/ projects/ delay_relay/

Delay relay

A simple, solid state delayed off  'relay' for 12V and 24V systems. You could use it to control a Webasto water pump or CB radio, or anything you want to be automatically switched off after a delay.

Circuit operation

The delay circuit is based on 4541 logic circuit. Despite being an old chip, it is sill available from several manufacturers, e.g. TI, NXP and Onsemi. The 4541 contains an oscillator and divider, so it has everything needed to generate longer delays than a circuit based on NE555. The schematic diagram is shown below.

12V delayed
      off relay schematic diagram

The circuit has two inputs. The '12V' is connected to a permanent +12V supply, such as car battery. The 'ACC' input is connected to a momentary +12V supply, which acts as control input to delay relay circuit. The load is connected to screw connector X2. The load is powered whenever the 'ACC' input is active. The load stays powered for the delay period and then switches off.

The circuit has many zener and signal diodes. Their purpose is to protect from all the nasty voltage events that could be encountered in a vehicle. For example, reverse battery polarity, jump start from a 24V battery and load dump. They aren't tested to be ISO 7637-2 proof, but the circuit should withstand practical scenarios pretty well.

The power switch is constructed from two PMOS transistors, which are integrated into one package. The benefit is reduced power consumption, size and contact wear elimination. Special attention has been paid to quiescent current. When the delay has elapsed, the power from CD4541B IC is completely cut off. So only the leakage current of the PMOS power switch is draining power, which is negligible.

You can select from four delay settings, spanning from approx. 7 seconds to almost half an hour. If none of the predefined settings suit your need, you could tweak C1,R1 and R2 values according to the equation shown in circuit diagram.


The PCB is designed with Cadsoft Eagle 5.12.0. It fits Hammond 1551RBK enclosure. Below is Eagle 3D rendition of the board. Input connector is shown on left and output connection at right. The LED lights whenever the load is powered. You must solder the SJ1 and SJ2 jumpers according to markings on silkscreen, depending on the delay length you need.
12V delayed off relay Eagle3D
Downloads for the design files, Gerber files and documentation are below. You can order bare boards from PCBWay using this link. Here is TME shared project which contains all the components for the project, except the PCB.

              Eagle .sch and .brd
delay_relay_v11.pdf             Circuit diagram, assembly drawing and BOM in PDF format
delay_relay_v11_gerber.zip  Gerber files

kair.us/ projects/ delay_relay/

page created 5.2.2022
last updated 20.2.2022 webmaster@kair.us