Additionally, if the logic is wrong or you want me to use a high side switch, I can do that for you. It will also help to know what kind of current the loads will be drawing.

 

You cannot run 12V inputs to a 74LS86. Don't use an 74LS86, use a CMOS gate, The 12V inputs are compatible with the CD4077B, and you only need a single transistor.

NOTE: Schematics without values are not much use.

Very interesting! I will have to read up some more

You cannot run 12V inputs to a 74LS86. Don't use an 74LS86, use a CMOS gate, The 12V inputs are compatible with the CD4077B, and you only need a single transistor.

NOTE: Schematics without values are not much use.

Well the values would depend on the devices yes, that has not yet been been determined, sorry to bother you guys.

 

Look. For what is probably the 3rd time: I KNOW YOU DO NOT NEED A CLOCK AND A COUNTER. It is there solely for the purpose of demonstrating the logic and the output driving capability. Just try to pretend it is not there -- OK?

 

Here is the same XOR logic, but this time I have a high side switch. The CD4077B output should be able to provide the 3mA of base current required to get 30 mA of collector current to the load.

Note: the purpose of the counter is to go through all the possible input combinations so that you can verify that the logic is correct. Because a single transistor will invert the logic, I have chosen a CMOS XNOR gate which also has a level of inversion the result is a positive logic XOR.

EDIT: One more thing, R1 is your LOAD impedance. DO NOT CONNECT YOUR LOAD IN PARALLEL WITH R3.

 

You can make an RC pulse detector to detect the Flash signal.
Flash signal overides brake signal. When no Flash signal, the circuit defaults to brake signal.

 

You can make an RC pulse detector to detect the Flash signal.
Flash signal overides brake signal. When no Flash signal, the circuit defaults to brake signal.

Something like this:

 

Here's is an efficient, power XOR gate that uses 4 power P-MOSFETS and 2 resistors.

Two MOSFETs are needed in series back-to-back to prevent back feed through the off MOSFET.
That works because MOSFETs conduct equally well in either direction when ON.

The P-MOSFETs can be just about any with a Vds max of 30V or more, and an on-resistance of less than 0.06Ω (to avoid having to add a heatsink).

 

Hi Mogman,

An off-the-shelf, 12 V DC 'true off-delay' timer should suffice.



When the turn-signal switch is actuated, the 'true off-delay' timer is enabled every time the flash voltage goes high and it's 'NC' contact opens and remains so even when the flash voltage goes low. Hence the lamp flashes and continues to do so even when the brake switch is actuated.

With the turn signal switch being turned off, the 'NC' contact closes and the lamp lights up continuously when the brake switch is actuated.

Nandu.

 

A 12 V DC 'true off-delay' timer should suffice.

You need to add a high-current diode so the brake voltage doesn't feed back to the turn signal line.

Edit: vu2nan edited his post #28 to add the diode.

 

Hi crutschow,

I'm sorry for the oversight.

I have added the diode.

Here's my edited schematic.



Thank you very much.

Nandu.

 

Corrected the missing attachment

My answer has been edited.

It is considered impolite to modify posts in a manner that makes a member's reply seem clueless.

I updated my post (#11) to reflect that you made an edit that made my post seem clueless.

 

Hi dl324,

I thank you for your feedback.

The following modifications have been carried out:

My post #28: Reverted to its original version.

My post #30: Edited schematic included.

Nandu.

 

You need to add a high-current diode so the brake voltage doesn't feed back to the turn signal line.

Edit: vu2nan edited his post #28 to add the diode.

Thank you, crutschow.

Nandu.

 

My post #28: Reverted to its original version.

My post #30: Edited schematic included.

It wasn't necessary to do that. Now I need to check the posted I edited as a result of your previous changes to see if the edit now seems clueless...

 

Remember that an incandescent lamp is low resistance until it heats up. So the starting current is something like 15 times the
running current. The fuse is slow compared to the lamp warmup time so it doesn't usually blow. Transistors are more
likely to blow. A 3 amp lamp might pull 45 amps at turnon (perhaps less). Depending on how fast the filament cools
you might get a surge at every lamp blink.

 

Remember that an incandescent lamp is low resistance until it heats up. So the starting current is something like 15 times the
running current. The fuse is slow compared to the lamp warmup time so it doesn't usually blow. Transistors are more
likely to blow.

True.
But the surge is an exponential with a time-constant of only about 4ms for a typical incandescent bulb, and the low on-resistance MOSFETs suggested for my circuit in post #27 should readily handle that surge without problem.
For example, for a 45A, 4ms TC exponential surge, the maximum energy dissipated in a 0.06Ω MOSFET on-resistance would is only about 0.3 Joules or 0.3 watt-second, according to my simulation.

Edit: After some further looking at MOSFET surge ratings it would appear that one with a large surge rating is needed to avoid possible MOSFET failure in driving the lamp.
Ones with a surge rating of at least 100A (for a 300µs pulse) should be used.