Momentary Latching Switch

Categorized | Learn To Play Rock Guitar

Posted on 14 June 2010. Tags: electronics, equipment, momentary latching switch, momentary latching switch circuit, momentary switch latching relay, mosfet, pedal, switch

Momentary Latching Switch

I need help making a circuit?

Hey everyone, i need alot of help with this.

I need to build an Electronic Switching circuit to control headlights on a motor vehicle with requirements as follows:
-The Head lights must start off in low beam when turned on.
-The Low beam bulbs are to be wired in parallel and will be 13.9v, 100 Watt bulbs.
-The high beam bulbs are to be wired in parallel and will also be 13.9v, 100 Watt bulbs.
-The circuit must automatically shut down 90 seconds after ignition is switched off, if it is still enabled.
-The latch signal, a trigger or dip switch between high & low, will be momentary short to earth and it must switch with an electronic cct, NOT A RELAY!!!!!

What are your idea on how to make this circuit??

I thought having the 555 timer attached to ignition and a Flip flop to the lights, but im not sure.
Please if you know how to, or have any ideas on this please help.
Or if you know of any forums or video which could help, please post.

thanks.

Not sure why you would avoid a relay, but anyway, taking your points...

- Power supply. For logic circuits. It makes sense to use +5V from a three terminal regulator like the 7805. Bring the 12V from the fuse through a series diode, a large shunt capacitor (1000uF) and the regulator device. The lamps will go from the fuse through the on/off switch to the lamp switch then to the lamp and then return to the battery negative. This cable has to be large enough to minimise voltage drop and carry the current (15A for 2 lamps, 30A for 4 lamps on together. Total drop for battery to fuse, fuse, on/off switch, cable and lamp switch and lamp sockets and return circuit to battery is 0.1V or 0.2V according to the lamp rating you give. WIth up to 4 lamps on, the current is 30A, so the allowable resistance is 6.6 milli-ohms. The location of the lamp switch is important.

- Reset. The headlight on/off switch activates the control circuit. This has a "master reset" circuit, so when powered up the state of the headlights is switched to low beam. The best way to do this is with a voltage comparator, and yes it could be a 555, but that is more tricky to get working properly. Consider the MAX6438 or look up "power on reset circuits". The object is to generate a brief pulse after the power has risen to its correct voltage. This resets the flip flop that controls hi or low beam to the low beam state.

- The lamps are 13.9V x 100W, so the operating current is 7.2A each, but there are either 2 or 4 lamps, so 30A. The surge at switch on is much higher, briefly, so the transistors used as switches must be able to handle this surge. Lets say that is 300A. For this reason I would chose power Mosfets for the lamp switches. Bipolar transistors will need additional power to drive the base circuit too. Mosfets only need voltage, no current. As N channel mosfets work better, cheaper, I suggest the lamps in the +12V side, and the Mosfet between the lamps and common (battery -).

- Chosing a suitable mosfet. See first link. This is demanding. Here in lies the problem, finding a suitable device for the switch. You need a >30V logic level N channel MOSFET with >50A current rating and < 3 milliohm on resistance. For the closest I found see second link. Note that this transistor needs 10V for the Vgs to turn on to 0.005 ohms though, and must be the TO220 pack. The voltage drop is 0.15V and the dissipation is 4.5W so it needs a heat sink. One solution is to use several of these in parallel on a heat sink. Also I worked on 4 bulbs, not knowing how many are on together. The more common IRFZ44N has a much higher on resistance, so could drop 30A x 0.028 ohms = 0.84V and dissipate 25W, so is not practical. For a single 50W bulb it looks better, but that is like 1/8 the current of this application.

The high and low beam are controlled by a flip flop, probably a D type 4013 is fine for this. D goes to Q bar, toggle (clock) by the beam switch using a momentary ground, BUT - must de-bounce the switch carefully. If it is a changeover switch, that is easy enough, and it can also switch the set and reset lines with a changeover switch. The power on reset is used to set or reset at switch-on. The output of this flip flop goes to the logic level switches, but needs an interface to convert the 5V logic to 10-15V gate drive. Ideally that will be clamped by a zener diode to prevent spikes above 20V which destroy the device.. This interface can use a small driver N channel MOSFET, with a 560-1000 ohm resistor to pull up the gate of the LL switch to 10-15V. See the third link, a good description of debouncing and the circuits on page 2. This will be important.

I have tried to give you the information to work out the details yourself. The power from a car battery is possible higher voltage than expected, but varies from 12-15V nominally, and can have large spikes and brownouts, mostly from the starter. The temperature in a car, especially the engine compartment, is high - not a great environment. In this setup the voltage drop will be an issue too, and probably de-bouncing.

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