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Automotive Time-Delay Relay

Status: Units are built to order - Please expect up to 1-week delay after receipt of order until order is shipped

Now available: A new version of the TDR (called TDR-T) which stays on as long as a trigger is present, up until a maximum time. This can be useful if, for instance, you want your dome light to turn off if you leave the door open.

Product change: The TDR-L, TDR-PL, TDR-ML, TDR-PML, TDR-MLC, TDR-PMLC, TDR-MLF, and TDR-PMLF have been replaced. The TDR, TDR-P, TDR-M, TDR-PM, TDR-MC, TDR-PMC, TDR-MF, and TDR-PMF now have selectable short and long delay ranges, so the long-delay versions of these relays are no longer needed.

Now available: A new version of the TDR (called TDR-MF) which turns on when the trigger goes away. This can be useful if, for instance, you need something to turn on after you turn your ignition off.

Now available: A new version of the TDR (called TDR-MC) which is triggered with a momentary switch or signal, and during the turn-off delay can be canceled by pressing the switch again.


Example Applications

For sale here is a compact solid-state 12V relay with adjustable turn-on and turn-off times, and two separate inputs. It is useful for many automotive applications, including H4/9007 HID retrofit systems and motorcycle HID retrofits, but it is also useful any place you would like to add a delayed relay in your vehicle, such as for powering your car windows for a short period after turning off your car or turning on a fan when the ignition is turned off. Remember, this relay isn't only useful for the applications listed above, it can be used for many different things.


  • Adjustable turn-on delay
  • Adjustable turn-off delay
  • Selectable delay range: short delay setting gives 0-120 second range,
    long delay setting gives 0-30 minute range.
    Setting applies to both turn-on and turn-off delays.
  • Can be triggered by positive or negative (ground) signal, selected by polarity switch
  • Dual isolated inputs
  • Simple 0.25" spade connectors
  • Solid-state operation for high reliability
  • Compact size: 2.375" x 1.61" x 0.81" (60 x 41 x 21 mm), or
    1.53" x 1.61" x 0.81" (39 x 41 x 21 mm) without the mounting ears
  • 10A maximum continuous load, 20A peak
  • 3mA standby current consumption (4.5mA for TDR-P* models)

Time-Delay Relay
Time-Delay Relays
Time-Delay Relay Adjustments
Time-Delay Relay Adjustments

This relay comes in a small plastic box with two mounting holes, and 5 standard spade connectors on the bottom. The electronics inside are encapsulated, or potted, in epoxy and are water resistant. There are two small adjustment knobs on the back to adjust the turn-on and turn-off delays. The five connections are:

  • +12V from battery (an inline fuse is required)
  • Ground
  • Load - to be connected to the positive connection of device if using positive-switching TDR-P, or to the ground connection of the device to be switched if using negative-switching TDR.

  • 1 & 2 - trigger inputs, to be connected to signals used for switching relay on and off

There are two versions of every TDR model, the TDR-P and the TDR. The positive-switching version, called TDR-P, switches the high side, or positive side. The standard TDR only switches the low side, meaning it switches the ground connection of the device (HID ballast or other), while the positive connection stays connected to battery voltage. Some applications cannot use the positive-switching scheme, so the TDR exists for customers who need negative or low-side switching, or find it more convenient for wiring. Most customers choose the positive-switching version.

Note that these relays are NOT fully waterproof. Because the connectors are standard spade terminals, which are exposed to the elements, too much water will cause them to short out; this is only preventable by using waterproof connectors. So be sure to mount these relays in a place where they will not get too wet, or be submerged. High up in the engine compartment, with the connectors pointing down and the back of the relay against sheetmetal, is generally safe, but if you wash your engine, be sure to disconnect the battery power and allow everything to dry before reconnecting the battery.

A packet of 5 matching terminals is included with each relay.

Time-Delay Timing Diagram
TDR Time-Delay Relay Timing Diagram
Time-Delay Relay Connections
TDR Time-Delay Relay Connections
Time-Delay Relay Connections
TDR-P Time-Delay Relay Connections
Time-Delay Relay Connections
Time-Delay Relay Connections

Triggering vs. Switching
One confusing issue with these relays is positive vs. negative switching and triggering. While these relays can use both positive AND negative triggering, they can only do positive OR negative switching: you must order the correct kind for your application. For the purposes of this write-up, "triggering" and "switching" are two different things.

"Triggering" is how the relay is activated, or triggered. When the relay receives a trigger signal on one of its two trigger inputs, it turns on after the time-delay. This trigger signal can either be positive, meaning a +12V signal, or negative, meaning a ground signal. The ability for the TDR to recognize either type of signal is useful because some applications need triggering from a switched +12V signal, while others (including many mechanical switches) require triggering from a switched ground signal.

"Switching" refers to how the relay switches power to whatever device is connected to it. The positive-switching TDR-P switches the positive side of a device, supplying +12V, while the ground connection is always connected to ground. The negative-switching TDR, on the other hand, switches the ground side of a device, so +12V power must be always connected to it, and the TDR switches the ground side on and off.

While it would be nice if the TDR could do both positive and negative switching, it's not technically feasible, so there are positive and negative versions of every TDR variant. Please be sure to order the correct kind after reading through the instructions and examples here.

The price for either the positive-switching Time-Delay Relay (TDR-P) or negative-switching TDR is $32, plus any applicable shipping charges. When you purchase using the PayPal checkout, it will calculate your shipping cost and should give you multiple options for shipping.

Interested? You can order one or a pair quickly and easily by clicking on one of the buttons below. Feel free to add more products to your cart before checking out, and save with combined shipping. If you would like to pay by money order, or have some other special needs, just email us and let us know how we can help you.

Other Information

Item Added!
Positive-Switching Time-Delay Relay (TDR-P)
Price: $32
Item Added!
Negative-Switching Time-Delay Relay (TDR)
Price: $32