can someone help me with an led switching circuit?

[badwhip]

I have a bi color led red/blue with a common cathode. the led will be regularly blinking with blue from a 12vdc pulsing circuit. when a sensor I have detects motion it has a wire and will send out a negative signal to the bi color led to switch it from blue to red as long as this negative signal is present. I figured I could use a couple of transistors to get this done but I'm not sure where to start. I know I will need to take this negative signal and convert it to positive for the positive side of the red led. I have 12vdc and ground as a power source.

on a side question how can I use a transistor in place of a mechanical relay that will switch with a ground command?

thanks to all that respond.

 

[dr pepper]

Welcome to the forum.
If you can make it out try this:

A npn and a pnp transistor, the pnp switches + to the red led using your - signal, then that switches on the npn which robs power from the blue led, note you need the 1k in series with the blue led as shown, even if there is allready one in the circuit driving the led.

 

[badwhip]

thanks for the reply I will get some transistors and give it a try.

what components do I need to add if I wanted that red led to be blinking?

 

[dr pepper]

Something like this:

https://www.electroniccircuitsdesign.com/555-timer-circuits/555-timer-led-flasher-circuit.html

You can get bi colour leds that flash, dont know about red/blue though.

 

[badwhip]

quick question on that timing diagram it shows two LEDs on it I only have one so I'm alittle confused how to hook it.

 

[badwhip]

I was able to figure it out. thanks

 

[dr pepper]

Omit top led and resistor, sounds like you sussed it.
Let us know how you get on.

 

[badwhip]

hi finally got around to hooking the circuit up and testing it. one thing I would like it to do that it is not. the blue led is blinking via the pulsating input. when I ground the one input for the red one it goes on but the blue also continues to blink. is there a way to get the blue led to cut off power when the red led is on? maybe another transistor to stop power to the blue led?

 

[dr pepper]

When you ground the input the blue led should stop flashing, check your connections around the bc547 and its 10k resistor.

 

[badwhip]

you are correct I had the one miss wired I got it working the way I asked. but the first way I wired it might work for me as well because it gave the illusion that the red led was flashing because the way the blue was going on and off but I think I will use this circuit then add the timer to pulse the red led.


on another issue I want to use a transistor instead of a relay to switch loads because transistor will draw less currant during holding can I use a 2n6387 or 2n6667 transistor to do this?
I will have a positive trigger to activate one relay but I also will need to use a negative trigger for another realy trigger.

 

[dr pepper]

A bipolar trans might use as much as a relay depending on what the load is, but a fet wouldnt, what do you want to switch?

 

[badwhip]

it's for car alarm use so the transistor would be used to break a circuit so it would be powered while car is off.

 

[dr pepper]

If your interfacing to a another circuit then consider a relay, a normally closed contact in a relay is on without the coil powered.