LED component help

Don’t know if this is the place to ask, but I have an LED that lights when a sensor provides ground. The sensor is very sensitive and will often provide ground, lighting the LED momentarily, and then it goes out because the sensor realizes it shouldn’t be providing ground. I’d like to cause a delay of, say 30 seconds, before the LED will light so that it will only come on when the sensor has been providing ground for more than 30 seconds. What component(s) would be required to accomplish my goal? Could you please provide a schematic drawn in scheme-it? Thanks

Hello @sb17l ,

Sounds like you might need an on-delay timer or a micro to complete this task. Do you have a current schematic from scheme-it you would be willing to share or details on the sensor you are using to turn your LED on? I am not sure I fully understand what is currently happening in your circuit.


I’m using the Gems XLS-1 Ultrasonic Level Sensor, part number 27815 (1/4" MPT Dry Sink). There’s a schematic on the data sheet found here:

There are only three wires on the sensor, one for power and ground (12VDC) and the other which provides ground to my 12 VDC powered LED (with embedded resistor).

My current application looks something like this (drawn on scheme-it):

An arrangement similar to that shown below might serve the purpose. U1 should be a (single supply capable) linear comparator suitable for use with the available supply voltage.

Thank you, Rick, that’s helpful. By any chance, could you please provide ohms values for the various resistors, and perhaps appropriate part numbers for the transistor, capacitor and comparator? That would really help. My circuit is powered by approximately 12 to 14 VDC and the LED that I’m lighting has an internal resistor to bring voltage down to appropriate level.

One more thing…your schematic shows current source on the top and far left, with a ground at the bottom but no output connection on the far right (coming from the comparator). How do i integrate your schematic into mine? In other words, can you please amend your schematic to include wiring of the XLS-1 and LED shown in mine?

I’ll politely decline to provide specifics, because A) exact values and component selections aren’t all that important, and B) it will prove immensely more instructive for you to work through such details on your own.

Your sensor’s output is of the current-sinking type, commonly called an “open collector” or “open drain” output. Connected to the input node at left, it will either conduct current from that node to ground or not, depending on the sensor state. When it does conduct, transistor Q1 is turned off, and C1 discharges through R2. When the input node is left to float, Q1 is turned on via R1, and C1 charges through Q1, R3, and D1. The voltage across C1 will therefore move toward Vcc or ground depending on the output state of your sensor, at a rate dictated the product of C1 with either R2 or R3, depending on the direction of travel.

The comparator compares the voltage across C1 with some fixed fraction of Vcc, determined by the ratio of R4 to R5. (Do a search for “resistor divider” if the concept is unfamiliar.) Many linear comparators are offed with an open-collector output, similar to that of your sensor and usable in the same ways.

Through the choice of values for C1 and R2-R5, one can establish different delays for transition of the output in response to changes in the input state, which are independent of supply voltage to a first approximation. LTspice is a no-cost tool useful for simulation of circuits such as this, which may prove helpful.