555 timer circuit help

[garry7263]

Re: 555 timer circuit help

S1 is a voltage controlled switch

My thought was that when the timer is on, it switches relay 1, sending current to the Voltage regulator and completing the circuit to light the glow plug and the the green LED (which is turned on by the voltage going through the switch. When that circuit is active, it operates relay 2, which turns off the red LED ( normal state is on so when the light circuit is active, it is off, and when the light is dead, the relay goes back to the on state.

I dont know how to use an op amp. I am more of a troubleshooter. I can find bad parts and repair them, but not very good at designing circuits.

[eccerr0r]

Re: 555 timer circuit help

What is S1? Is it a optoisolator? If you actually want to light up the light bulb, 3A needs to pass through the bulb, and by KCL, 3A needs to pass through S1 +/- terminals because it's in series. As budm indicates, and if it's an LED in there, it will smoke as its resistance is higher and thus all the voltage will drop across it instead of the bulb you want to light up. (actually it probably won't smoke with only 1.5V... but that's still a problem as your light bulb won't light.)

As said, without using op amps, I don't think there's really anything that will have low enough drop in series with the light bulb such that it will have 1.5V and still have enough voltage to turn on an LED.

As you draw there:
S1's +/- will take the whole 1.5V. It will not pass enough current to light the bulb. Lamp will be always OFF.
Thus S1 will be on whenever relay 1 is on, the Green LED will be on when relay 1 is on, as long as the bulb filament is intact.
Relay 2 will be on whenever relay 1 is on and hence LED2 will always be on when relay 1 is on.

[garry7263]

Re: 555 timer circuit help

The idea is to push the button and have the light come on for 10 seconds, with a green LED indicator to tell if the lamp is lit. If the filament is not working, the red LED will indicate that the filament is not working (broken). The light is actually a glow plug on a radio controlled helicopter. Just push the button and it lights the glow plug for 10 seconds to give me time to hit it with the starter. The reason for the LEDs is so that if the filament in the glow plug is good, it will light green and if it isn't, it lights red (letting me know I need to replace the plug without having to pull the plug and check it visually).

I want to use a timer so that I can draw off the receiver battery but have it turn off on its own so there is no chance of forgetting to flip a switch manually for fear or draining the battery needed for controlling it.

I use the term "light" because basically it is the same thing. I am going by 3 amps on what others have said glow plugs draw. The voltage is nominally 1.5v to keep the filament from burning out (though some use 2.0 volts), so I think a regulator would be the best way to step down the voltage. Maybe using diodes to lower it to a safe 1.5v? I have seen some 1.5v 5 amp SMDs (or maybe a 1.8v). I am sure there are multiple ways this could be done, its just trying to keep it as small and light as possible.

[garry7263]

Re: 555 timer circuit help

Here is a schematic idea

Im not constrained to anything in particular other than wanting it to be as small and light as possible. Keeping it to SMD components and the board as small as possible.

[eccerr0r]

Re: 555 timer circuit help

Yep, changing the constraints will make it easier. Using a photodiode or light dependent resistor sensor may simplify things.

[budm]

Re: 555 timer circuit help

"The failure LED would only come on if the button is pushed, " So basically this button is for the lamp test switch, right? If that is the case why not make it so if you push the Lamp test switch and the lamp does not come on then you know you have open lamp circuit?

[garry7263]

Re: 555 timer circuit help

Having trouble using something to make a schematic. Cant find the components. The one I used before had expired. I will see what I can find so I can post a schematic or just draw it out and take a picture.

[eccerr0r]

Re: 555 timer circuit help

At first guess I don't think there's a simple solution given the (implicit) design constraints...

- 5V supply for timer
- use only transistors and other passives
- lamp is 1.5V 3A
- use separate 1.5V supply for lamp
- relay or huge mosfet to switch lamp is acceptable
- go/no-go lamp check *only after button press*
- minimum size

I'm thinking an op amp is needed just because of the low lamp operating voltages involved. Diode drop detections with BJTs aren't acceptable.

Maybe someone can think of a solution without an op amp or its equivalent circuit.

[budm]

Re: 555 timer circuit help

Are you going to use Incandescent lamp/ You will have to figure out the inrush current due to the cold resistance of the Incandescent lamp. Cold resistance can be 10~15 time LESS than the HOT resistance.
1.5V lamp with 3A = 4.5W, so the hot resistance will be 1.5V/3A = 0.5 Ohms, so cold resistance will be 0.05 Ohms (at 10 time less), so inrush current will be VERY large!

BTW: 'I figure that the lamp itself could be the resistor for the green LED' you are not going to be able to force 3A through that small Green LED indicator, you will see nice smoke from the LED.
Schematic of what you are doing will help.

[garry7263]

Re: 555 timer circuit help

The failure LED would only come on if the button is pushed, so that it either works and shows the green LED, or shows the red LED meaning the filament isnt working.

I would like to keep it confined to SMD components and make it as small and light as I can. I figured that I would probably need to use pin 3 to trigger a relay to power a voltage regulator to draw that kind of current.

I was only thinking of using the filament in place of a resistor to save space on a PCB. There are small enough resistors that space would not be an issue. So I guess that after triggered, the power to the filament could be separate from the indicator LEDs. On the negative side of it, could have 2 transistors, one (for green LED) in an on status when the filament is good, but turns off the LED when the filament breaks. One in the off status that when the current on the negative leg of the filament loses power, it triggers the red LED to come on. I don't have a lot of experience in circuit building and especially with transistors.

[eccerr0r]

Re: 555 timer circuit help

The 555 is a pretty powerful chip but cannot handle 3 amps. IIRC it has a maximum output of about 200mA, and I don't think it can pass that at 5 volts.

Plus you have that LED in series which means 3 amps must pass through that due to KCL. You'll need a relay or transistor to pass that much current.

How are you detecting the filament failure? Should the filament failure LED be on, even when the button isn't pressed?

A schematic would be helpful.

[garry7263]

I am trying to build a light lamp that draws about 3 amps at 1.5 volts that uses a 555 timer to trigger it on for about 10 seconds. It needs to also include two LEDs, one green that tells when the lap filament is working and a red one that tells when the filament is broken.

The power source is around 5 volts and I figure that the lamp itself could be the resistor for the green LED. So far the circuit I have is the positive going to 4 and 8 pin, the 1 pin going to ground, the 2 pin is going to positive with a 100k resistor and a switch going to ground, pins 6 and 7 have a 500k resistor going to the positive leg and a 10uf going to negative for the time period. Pin 3 goes to the bulb and then to the green LED, then connected to ground.

A few questions: First, should I connect pin 5 with a capacitor to ground? What would be the voltage coming off pin 3? Could it handle 3 amps? Im guessing about 2ish volts to the bulb. I was thinking using a 1.5 volt regulator from pin 3 to go to the bulb. I don't think there is any problem with the circuit to that point. I was thinking of how to trigger on the red LED and turn off the green one when the filament is bad. My guess would be to use two transistors, but not sure how to trigger them.

One thought I had was to use a relay triggered by pin 3 to draw directly from the 5v in to power a secondary circuit that would be able to better handle the amperage. Still unsure how to make the LEDs trigger in relation with the bulb filament.

Basically, the circuit is: you push the momentary switch, and it powers the lamp and green LED for 10 seconds (LED indicating that the lamp is lit and working), or triggers the red LED if the filament is broken.

Any help would be greatly appreciated. Thank you in advance.