I am doing something that I am sure has been done a million times by a million people. I am trying to wire up a 555 timer in a monostable configuration to be used on an aquarium top off system. The float switch will pull the trigger to ground which will start a pump that will run for a few seconds and then shut off. I was trying it out with a small 12V cooling fan. It worked 4 times and then on the fifth time the fan came on for a quick second and then a few seconds later there was smoke and a glowing ground pin on the 555 timer. Can someone help me to figure out where I went wrong? This is my schematic which is 90% ripped off from a schematic that I found online and the other 10% is the n channel mosfet and the motor and flyback diode. I did forget to add the flyback diode on my test runs if that makes a difference.

 

Any chance you got the MOSFET pins wrong? It's really easy to do and a very commonly seen boo boo. I think if you are especially unlucky, the MOSFET body diode could route all current through the 555 instead of through the MOSFET. Hard to explain the glowing hot ground pin otherwise, unless there's a problem with the actual build.

 

Any chance you got the MOSFET pins wrong? It's really easy to do and a very commonly seen boo boo. I think if you are especially unlucky, the MOSFET body diode could route all current through the 555 instead of through the MOSFET. Hard to explain the glowing hot ground pin otherwise, unless there's a problem with the actual build.

Did anything look wrong with the schematic? It is my first.

 

I am doing something that I am sure has been done a million times by a million people. I am trying to wire up a 555 timer in a monostable configuration to be used on an aquarium top off system. The float switch will pull the trigger to ground which will start a pump that will run for a few seconds and then shut off. I was trying it out with a small 12V cooling fan. It worked 4 times and then on the fifth time the fan came on for a quick second and then a few seconds later there was smoke and a glowing ground pin on the 555 timer. Can someone help me to figure out where I went wrong? This is my schematic which is 90% ripped off from a schematic that I found online and the other 10% is the n channel mosfet and the motor and flyback diode. I did forget to add the flyback diode on my test runs if that makes a difference.

The flyback diode is Very Important. This is the reason the circuit blew up. Place the flyback diode and also a 47R resistor between the gate and the output of the 555. What is more place a 47K resistor from the MOSFET gate to ground.

 

It looked OK to me, but that may not mean much, as I'm no 555 expert. What I was suggesting is that the circuit may be fine, but there is a big gap between paper and reality, and little things matter. Maybe post a picture of your build.

 

The flyback diode is Very Important. This is the reason the circuit blew up. Place the flyback diode and also a 47R resistor between the gate and the output of the 555. What is more place a 47K resistor from the MOSFET gate to ground.

I thought the 555 is supposed to sink current when the output is low. Why the need for the 47K pulldown on the mosfet gate?

 

I guess I need to check over my build again and implement the changes suggested by mik3.

 

Just throwing this out as a question. Is there a limit to the size of capacitor that you can use in the RC circuit? I notice from reading the datasheet that when the capacitor voltage reaches 2/3rds of Vcc, the output goes low and the discharge pin provides a low impedance path to ground to discharge the capacitor and ready it for the next cycle. I was thinking that if the capacitor is very large the current through it to ground may be enough to burn up the IC. I am using a 220 uF capacitor which doesn't seem like it would be very big but what do I know.

 

I don't believe that could make your ground pin hot, not even a direct short across the poles of the cap would do that, except maybe for a very thin wire.

 

I thought the 555 is supposed to sink current when the output is low. Why the need for the 47K pulldown on the mosfet gate?

There is the possibility the 555 to fail and the gate to be left floating. This might cause the MOSFET to operate in the active region. It will overheat and burn. The 47K ensures that MOSFET will be off.

 

Just throwing this out as a question. Is there a limit to the size of capacitor that you can use in the RC circuit? I notice from reading the datasheet that when the capacitor voltage reaches 2/3rds of Vcc, the output goes low and the discharge pin provides a low impedance path to ground to discharge the capacitor and ready it for the next cycle. I was thinking that if the capacitor is very large the current through it to ground may be enough to burn up the IC. I am using a 220 uF capacitor which doesn't seem like it would be very big but what do I know.

I am not sure if they mention that in the datasheet but it might be a possibility.
Use a 100K and a 22uF. Also, you can put a 10R in series with the discharge pin to limit the discharge current.

 

I am not sure if they mention that in the datasheet but it might be a possibility.
Use a 100K and a 22uF. Also, you can put a 10R in series with the discharge pin to limit the discharge current.

I think I will try it again with your initial suggestions. I skipped the flyback diode because I thought that given that the load was a brushless motor and was being commutated via circuitry inside the fan that the internal driver circuit would have its own back emf protection on each of the coils. I have one more 555 timer that I will plug in but I will admit I am a bit nervous. This blowup was pretty spectacular

One more question, would the fact that the 555 timer that I used was one of the CMOS varieties make a difference?

 

Motors are always inductors, and should be treated as such. The basic schematic looks sound.

I have used will into the 1000's of µF with no problem. You don't have a input signal conditioner, but the duration of the pulse means it should not be a problem. I calculate 2.4 seconds.

This is not too dissimilar to the article I wrote for monostables.

555 Monostable

 

I know you have put quite a bit of time in to this circuit, and you'll probably stay with this design; but may I suggest you take a look at this thread.

 

Motors are always inductors, and should be treated as such. The basic schematic looks sound.



I have used will into the 1000's of µF with no problem. You don't have a input signal conditioner, but the duration of the pulse means it should not be a problem. I calculate 2.4 seconds.

This is not too dissimilar to the article I wrote for monostables.

555 Monostable

Thanks for the link to your thread Bill! That helped a lot! Can you explain a bit more about the need for a signal conditioner and its relationship to the pulse duration? I read in your thread that the absence of it can create an illegal condition if the trigger is held low past the timeout period. I was just using a wire connected to ground as my switch and since this will eventually be hooked to a float switch it very well may be held low for a long period of time and I can't remember if I may have been holding the switch closed to test that condition. The 555 is just to keep the pump on for a minimum amount of time to get rid of the rapid cycling of the pump when there is a ripple on the surface of the water.

 

I know you have put quite a bit of time in to this circuit, and you'll probably stay with this design; but may I suggest you take a look at this thread.

Wow that is a long thread. Thanks for linking it and I will work my way through it. At first glance it looks like a much more complicated solution than what I actually need. My top off doesn't need to be precise because it is just keeping water topped up in a sump under a fish tank. The pump is a 12V DC motor running a peristaltic pump and it just needs to turn on and off at the appropriate time but it does not need to be precise. The 555 timer is just designed to eliminate the quick succession of cycles on the pump that occurs when a ripple on the waters surface jiggles the float switch.

 

Basically if you hold the SW on pin 2 down it will keep the timer on even after timeout. Adding the RC network (aka, signal conditioner) only sends a pulse to pin 2, instead of a continuous ground. This means the timer will time out normally no matter what. Basically it is for the human part of engineering.

I've been feeling pretty depressed lately, and it has affected my site time. Another thread for a similar yet different project for a saltwater aquarium, you can save yourself time by working your way from the back to the front, but if you want to understand it you have to go from the front to back, as it was designed and tweaked for the OP on demand.

http://forum.allaboutcircuits.com/showthread.php?t=24179

In his design it would start to fill if the float switch closed (the level was low). If this condition lasted too long it would alarm both audible and visual and shut the pump down.

 

Something else, you could have done what I have done many times, a dumb wiring error. It can happen to all of us. If you have a digital camera or a flat bed scanner why not scan both sides of the board?

 

Something else, you could have done what I have done many times, a dumb wiring error. It can happen to all of us. If you have a digital camera or a flat bed scanner why not scan both sides of the board?

I went over the connections last night and everything seemed good. I will clean it up and then photograph it. I am guessing one of the on off cycles fried the 555 due to my lack of back emf protection. I'm using a 12V SLA battery for power. All connections to the positive rail (except for Vcc) are through 10K to 15K resistors which should limit current to the device to no more than 1.2 milliamps. Vcc is connected directly to the 12V rail which in my mind is the only pin that could receive a large amount of current but looking at the block diagram it should have plenty of limiting internally within the chip (I guess unless a voltage spike fused something on the chip).

 

Sockets are your friend, unless it is in a high vibration area.