Separate Brake Light From Turn Signal

I know its a little late, but here is an alternate circuit.
This uses 1 IC (U1A/U1B, an CD14538 dual timer), two NPN transistors and 3 diode gates.
U1A and U1B are configured as missing pulse detectors to detect a pulse period of 2 Hz.
I've show incandescent bulbs, but LED's can be used.

The waveform graph shows the brake light status in the following sequence: 1)left turn then brake, 2)right turn then brake, 3) emergency flash.



 
eTech said:
I know its a little late, but here is an alternate circuit.
Click to expand...
Not an issue. I am so appreciative of this group. I will reproduce this one as well. I see your version has a separate power supply. As common as that is a 4 pin trailer harness does not have an accessory wire. The 5 & 7 do though and my truck has 5 & 7. The 3rd tail light has about 15 leds with a low current draw of about 200mA. Id like to know what M1 is.
 

Here is a revised version. I've changed the diode "OR" gates to CMOS (CD4071B). The diode gates are sensitive to supply voltage changes. This means there are two IC's in the design now. There is a possibility of replacing the OR/AND gates with a single combinational logic IC (CD4048B), keeping the IC count to two, but I think the diode AND will be ok.

M1 is the IRF7406 mosfet. I'm assuming the 3rd tail light already has circuity to drive the 15 LED's and only requires a 12V input.

I've show only the circuitry and changed the external input connections as an attempt to match post #1. The connector in post 1 shows a +12 connection, this is +BAT in my schematic. I think you should consider a +12 supply wire to feed whatever circuit you decide to use. This way, the circuit will be initialized and ready to operate.

The NPN transistors at the inputs is an attempt to "ruggedize" the CMOS input circuitry.

The circuit below works fine in simulation, but needs a bench test.

 
It was quickly determined that the schematic in post #1 was incorrect for what I wanted to accomplish. In fact I found out that it did the opposite. It was intended for hot rods with little room on the fenders for lighting.

You make a good point with the constant voltage.
eTech said:
M1 is the IRF7406 mosfet. I'm assuming the 3rd tail light already has circuity to drive the 15 LED's and only requires a 12V input.
Click to expand...
yes. This tail light is ready to use. I've been told some third brake light configurations can be quite elaborate and require over an amp or even two so I can appreciate the benefits that FR307 and IRF7406 add to this circuit. The 3904 NPN is nice to see too. I am wondering how do you bench test a circuit with SMDs in the mix?
 
ThomsCircuit said:
I am wondering how do you bench test a circuit with SMDs in the mix?
Click to expand...

Everyone has their own method. But for me, the parts are really inexpensive, so I would use a solderless breadboard and equivalent DIP parts. If an SMD chip didn't have an equivalent DIP part, then I'd use a SMD to DIP adapter. Thru hole passives (cap,resistor,diode) are usually not a problem to obtain. For the final product, use whatever you wish.

Also, keep in mind that the RC timing components have "estimated" values. Nothing has been written in previous posts regarding the flash rate (how fast the lights blink), unless I missed it. I used a frequency of 0.5 Hz, 50% duty cycle (or 1 sec on/1 sec off) for my design. The RC component values will need to be "tuned" for the final circuit.
 
"The Department of Transportation generally sets the speed at which a vehicle's turn signal flashes at 120 times per minute." My Tundra cycles every .70 seconds. 85 times per minute. The light is on for .35 and off for .35 seconds. Can it be tuned with a potentiometer?
 
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Yes. You can replace Rx1,Rx2 each with a good quality 500k linear trim pot. Set pot to middle. Turn on flasher, adjust pot until the third brake light shuts off. Measure pot resistance, then replace pot with resistor or leave in.
 
eTech said:
You can replace Rx1,Rx2 each with a good quality 500k linear trim pot.
Click to expand...
Alright, two linear, 500k trim pots will replace Rx1 & Rx2. Set to 250k. Turn one turn signal on and adjust that pot.
I'm doing this with all through hole components as I have all of the resistors, transistors, and caps in TH.
Can you recommend a sub for the Mosfet

I located this dual taper 500k
 
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Alright located all diodes.
---------------------------
1N5822 Schottky 3A 40V
1N5818 Schottky 1A 30V
1N4746 Zener 1W 18V
I would like to use a mosfet in a TO-220. I found this IRF5305 its 55v with a high amperage of 31. I'm not happy with this as the amps are way off. I'm just not having any luck in finding a suitable mosfet that has the important attributes. I've used DigiKey to find equivalents but they are all surface mount.
 
The IRF5305 MOSFET should work fine.
A high amp rating just means that's the maximum it can carry,
It will work from that amperage down to zero amps without a problem.
 
Hi

Here is a version using the CD4048B OR/AND function.



EDIT: Added U1A/B pin numbers.



Diodes in Post #90 look OK.
I concur with Crutschow post #91.
 
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Cx1 and Cx2 should be identical and have low tolerances for a dual pot to work.
Each pot of the unit should be wired like a variable resistor with connections configured such that BOTH increase and decrease resistance with the same direction of rotation.
 
eTech said:
Thru hole passives (cap,resistor,diode) are usually not a problem to obtain. For the final product, use whatever you wish.
Click to expand...
Thank you.
I am going to assume that D7 & D8 are 3A based on where they are in the circuit. But that's my logical assumption.
eTech said:
Diodes in Post #90 look OK.
I concur with Crutschow post #91.
Click to expand...
Thank you.
Did I interpret the taper pot instructions correctly? #89
 
ThomsCircuit said:
Hi, I created the schematic and PCB. How did I do?
View attachment 145991
View attachment 145989

View attachment 145990
Click to expand...

Hi
Schematic comments:
1. Q1 Mosfet should be a PMOS mosfet.
2. The circuit needs a terminal to serve as a ground connection.

PCB comments:
In general, layout looks ok. However...

1. The mosfet routing doesn't look right. Pins might be swapped, please double check.
In addition, the source and drain should be wide traces and a wide track run from D8 to the mosfet source pin. V+ to D8 should also be a wide track. These are carrying a large current to the brake LED.

2. What size track width did you use?

3. The PCB needs a terminal to serve as a ground connection.

4. The PCB needs mounting holes.
 
Thank you!

IRF5305: Design Spark uses the wrong symbol. I edited the symbol and checked the pin outs. The two fets happened to be correct in pinout.
The traces are 30 and I set the mosfet traces to 50. There is a ground pin and I also added the GND net to one of the mounting holes. There is a GND pour on the bottom layer.
Hope you like Italian.





SCHEMATIC. Detail Mosfet IRF5305
 
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