simple led switch, kinda

[lenberman]

I sometimes forget to take my cell in the car with me and I want to make a two transistor switch. The circuit must light a reminder LED when the car is started requiring me to touch a momentary switch to turn off the LED. 30 years ago I used to do a lot of hobby transistor breadboard projects so I thought if one of you guys could sketch a simple circuit I could follow it and I'd stop leaving home without my phone. Thanks in advance.
Len Berman
San Diego, CA

 

[Mickster]

It's not exactly what you asked for, but here's some ideas that could help to remind you without having to make anything.

1. Always put your keys and your phone beside each other. When I go to bed, my phone and keys are in the same place, ready for the morning run. When I come in for the day or night, and am still going to be using the phone, when finally finished, the phone goes beside the keys. Always.

2. Have a 12V-to-USB charger with an LED always in the vehicle cig lighter/power outlet. This output is almost always controlled by the accessory/ignition circuit and will only be lit up when you get into and start the vehicle.

3. Leave yourself a note. I tend to leave myself a reminder note, that lunch is in the refrigerator, on top of my keys. If I have to move the note out of the way to get to the keys......

 

[Pommie]

Do the adult version of Head, Shoulders, Knees and Toes - it's Wallet, Glasses, Keys and Phone.

Mike.

 

[AnalogKid]

If you search for 'transistor flip flop circuit' you will get lotsa schematics, mostly along the same theme.

**broken link removed**

These cover the basic idea, but you need a modification so the circuit always powers up in one state. This can be done with the right size capacitor replacing the left switch. Also, the left LED can be deleted and the resistor connected directly to Vcc like R1. I'll try to crank out a schematic specifically for your application.

I applaud your wanting to use transistors. Other options are a single CMOS flipflop chip like the CD4013, a two-gate flipflop circuit with a CD4093, a 555, and or course a microcontroller like a PIC or Arduino. But where's the fun?

ak

 

[gophert]

If you search for 'transistor flip flop circuit' you will get lotsa schematics, mostly along the same theme.

**broken link removed**

These cover the basic idea, but you need a modification so the circuit always powers up in one state. This can be done with the right size capacitor replacing the left switch. Also, the left LED can be deleted and the resistor connected directly to Vcc like R1. I'll try to crank out a schematic specifically for your application.

I applaud your wanting to use transistors. Other options are a single CMOS flipflop chip like the CD4013, a two-gate flipflop circuit with a CD4093, a 555, and or course a microcontroller like a PIC or Arduino. But where's the fun?

ak

The 555 timer option is super easy, cheap and no programmer.
It really depends on the vehicle brand used. If you have a vehicle where the lighter/USB power are disabled when the car is off would be easy since a simple 555 one-shot will trigger on power-up.

It will be more difficult if your vehicle has no way to know what is going on (12v socket (lighter) is always on. Most import vehicles are switched with the ignition. I haven't driven a Ford or GM lately but my last Ford was always on.

 

[Pommie]

I remember a circuit that used the door light switch to trigger a thyristor to activate a buzzer. A push button in the circuit would break the circuit to silence the buzzer. Do negative triggered thyristors exist or must it have had a transistor as an invertor?

Mike.

 

[AnalogKid]

Other options include a SPDT relay (no other parts, just the relay and a switch), an SCR circuit (mentioned above), ...

ak