Hi - I started researching a 555 based circuit design for a tank level alarm for a tractor spray tank. I need to avoid running the pump dry which would mean a $$$ repair bill. I started off thinking I would do this with an Arduino but I think I can do it simpler with a couple of 555's - one in bistable mode and the other in monostable mode.

I mocked up this circuit with a 555 I had lying around:

and it works as described. The trigger in my case would be a float switch mounted inside the tank. I realise this won't work as a single 555 circuit because as soon as I reset the 555 to stop the piezo and flashing LED the 555 would go straight back into alarm mode (float still in same position). I am not sure where to go from here. I need some way of only re-arming the 555 when the float returns to the upper position.

The float I am using is this one:

I can sense when the float has returned to the upper level (brown + black) and the trigger at the low level (blue + black). I am thinking if I have a second 555 in monostable mode that triggers when the float is high, the output would then power the bistable 555 that would then detect the low level of the float. If this is correct I am not sure how the 'mute alarm (reset)' function would work.

Any help is appreciated.

 

Why not try using the 555 in the Astable mode? Use the Float to arm the Threshold and Trigger terminals instead of Reset and Trigger.

 

OK - I will have to do some more reading. I still need to be able to 'silence' the alarm once it goes off. It may be hours before the tank is refilled, i.e. unit the float is back up to the top.

 

The Threshold and Trigger terminals act as a Set and Reset of a SR flip flop. It should work.

http://www.daenotes.com/electronics/digital-electronics/555-timer-ic-stable-multivibrator

The only difference is that instead of a Capacitor Charging and Discharging, you have a Tank that Fills and Empties

 

I don't quite understand how running the 555 in astable mode will do what I need it to do. Bear with me, I am new to this. The output in astable mode is square wave yes ? How do I use a square wave output to illuminate a visual indicator and run an audible alert ? Aside from that, I have found a simple circuit diagram:



I dont understand where my float switch fits in here, the float is essentially a DPST switch.

 

Sorry - by DPST I mean this:

 

A 555 by itself is not going to do any good if your tank will take hours to refill. In general, the 555 by itself will not help you if you need more than a minute. You will need some type of counter chip (7493 for example) that can count off how many 555 cycles occurs while your tank is in the low condition and then alarm if too many 555 cycles occur.

 

So the requirement is as follows. I have a tractor with a 500l spray tank. I can't allow the herbicide to run low and cause the pump to run with no liquid in it. The easiest way to sense the level is with a float switch. It can give me an upper reading and a lower reading. I could easily do this with an ATMega micro controller but I thought I would have a go at doing it with discrete components. The reference to hours to refill was more about that fact that once the tank is run to the low point I need to be able to silence the alarm and then be able to reset it, either by refilling the tank or by pressing a button to 'arm' the alarm again.

 

Sorry - by DPST I mean this:

View attachment 90189

That would be a SPDT.

 

So the requirement is as follows. I have a tractor with a 500l spray tank. I can't allow the herbicide to run low and cause the pump to run with no liquid in it. The easiest way to sense the level is with a float switch. It can give me an upper reading and a lower reading.

Does that mean that you are willing to install two separate float switches; one that senses tank full; the other that senses tank empty? One float switch will not do that.

I could easily do this with an ATMega micro controller but I thought I would have a go at doing it with discrete components. The reference to hours to refill was more about that fact that once the tank is run to the low point I need to be able to silence the alarm and then be able to reset it, either by refilling the tank or by pressing a button to 'arm' the alarm again.

If you add an On-Off toggle switch, it can be used to silence the alarm by removing all power from the entire circuit, provided that you remember to rearm the circuit the next time you go spraying. This has the advantage of not stealing power from the tractor battery when the tractor is parked.

Another option is to use a momentary push switch to silence the alarm. Use a Latching circuit that is set by the button, and that is cleared by the tractor ignition switch being turned off (tractor parked). The Alarm sounds only when Latch=0 AND Level=Low. Alarm is silenced by setting Latch to 1.

Next time tractor is started, and the tank has been filled in the interim, no Alarm. Start spraying, and the alarm goes off when the tank level drops automatically. Push the button, setting the Latch, silence the alarm. Drive the tractor to where you refill it, shut it down, refill the tank, restart it, and the sequence is armed for the next cycle. If you park the tractor without refilling the tank first, the next time you start it, you will get the alarm to remind you that the tank needs filling....

Note that this idea requires one 555 (used as a set-reset latch) and a different one to make the sound (astable).

State Transition Diagram:



View attachment 90192

 

The following circuit resets when momentary button is pressed (even when tank is still low) to turn off alarm & red LED. Momentary switch is on far right.

The float switch will light the green LED when tank is full.

The float switch will light the RED LED and buzzer when low.

It will keep the red LED lit the whole time the tank is at a low level (until the reset button is pressed).

Float switch and wire colors are highlighted. green LED is on far left.

 

I can't draw a schematic right now, so this will be all text. ow about this -

One 555 astable driving the LED and piezo. Normal astable circuit except that the reset pin is driven by a flipflop.

One CD4013 dual flipflop, only 1/2 used. The Set input goes to a R-C network to act as a power on set to make sure things come up in the correct state. The Reset input goes to your silence pushbutton switch. The Q output drives the 555 Reset input.

Now the fun part. Power for the entire circuit goes through the float switch blue and black; brown is unused. When the tank is low, the float switch applies power and the lights an sirens area ablazing. You hit the SHUT UP switch to quiet the din. When the tank is full, power is removed from everything, including the power-on-set R-C circuit. Repeat.

Or, both the flipflop and the astable can be done with one CD4093 quad NAND gate and an external transistor. Fewer parts.

ak

 

Does the tank have anti-slosh baffles? If not, a float switch will give erratic indications of liquid level when driving over uneven ground or accelerating/decelerating/turning. Some form of averaging/integration of its signals might then be needed.

 

This circuit might work but might need to change the input to cover alex_t comment.
I also wasn't sure if the float contact would closed or open when empty.
Anyway, I've shown it closed when empty.


OOPs...didn't realize already circuits posted..sorry

 

That would be a SPDT.

How embarrassing

 

Thanks for all the replies. I'm going to go and grab a pile of components today from Jaycar and have a play.

 

Does the tank have anti-slosh baffles? If not, a float switch will give erratic indications of liquid level when driving over uneven ground or accelerating/decelerating/turning. Some form of averaging/integration of its signals might then be needed.

The float switch i mentioned earlier has a very wide angle across which it is either on / off. I can't imagine any amount of sloshing that could take it from on to off.

 

Does that mean that you are willing to install two separate float switches; one that senses tank full; the other that senses tank empty? One float switch will not do that.

This float switch does exactly that. It is essentially a SPDT switch.

 

This float switch does exactly that. It is essentially a SPDT switch.

I guess the question is, is there a dead zone where neither of the wires (Blue or brown) are connected to black.

 

I guess the question is, is there a dead zone where neither of the wires (Blue or brown) are connected to black.

Yes - there is an angle of about 60 degrees where neither blue or brown are connected to black