Model engineer needs help

Hello everyone, I'm a model engineer who needs your expert help.
I have a steam powered model boat and I am trying to design a system that monitors the water level in the boiler and tops it up as necessary.
I have designed a sensor that detects water in a sight glass. This sensor gives an output of 3v when water is detected and 2v when no water is detected. This output is fed to an opamp (LM358) which outputs either 5v or 0v.
I also have a servo driver which gives either full clockwise rotation or full anticlockwise rotation of a servo depending on position of SW1. The values of R3 and R4 determine the amount of rotation.
What I want to do is use the output of the sensor circuit to control the servo driver.
I would also like to have a time delay so that when the sensor output changes from +2 to +3 there is a delay of about 10 seconds before the output of LM358 changes from high to low.
Does anyone have any ideas how to combine these two circuits?

 

Thanks Steve,
A 555 monostable circuit looks ideal for the time delay but this raises a few questions, (I'm pretty much an electronics novice).
1: can the servo driver be adapted to make it a monostable circuit or do I have to use a seperate 555? The servo driver produces either a 1mS or a 2 mS pulse every 20mS.
2: can I connect the output of the LM358 directly to pin 2 of the monostable circuit and if so is the 10k resistor still needed?
3: will the output from the monostable circuit remain high for as long as the output of the LM358 remains low + the time delay, or will the time delay kick in as soon as the LM358 output goes low and switch off whether the LM358 output is low or high?
4: if I connect pin 3 of the monostable circuit to the junction of R3 and R4 in the servo driver circuit will it effectively bypass R3 or does the 555 have any internal resistance which will need to be factored in?

Sorry about asking so many questions but as I said earlier I am a novice, however I find electronics fascinating and I'm learning something new every day.
Stew

 

Hi Steve
I would like to thank you for all the help and advice that you have given me so far, I really appreciate it.
I won't be able to breadboard it until tomorrow, work stops play.
I would like your thoughts on the circuit below, is there anything obvious that I've done wrong or missed?
I know I may have to alter R3 and R4 to give me the required servo movement.
Is 470 a suitable value for R8?
Is there any reason why C2 and C4 are different values, the monostable circuit and servo driver are circuits I've downloaded and I've no idea how these values have been calculated. If I could use the same value for both what would be the best value to use?
Cheers Stew

 

First of all let me thank Rleo for joining in.
I'll give a brief description of what I want the circuit to do.
When the water level drops below the sensor I want the servo to rotate fully in one direction to operate a water pump which refills the boiler.
When the sensor detects the water again I want the pump to continue for another 10 seconds before the servo turns fully in the opposite direction turning off the pump. This is to take the water level above the sensor so that the pump is not rapidly turning on and off when the water is level with the sensor.
The servo is the type used in radio controlled models to control steering, throttle, control surfaces etc. and is bsically an electric motor which alters the position of an arm. The position is determined by the length of an electric pulse. A 1mS pulse turns the arm fully in one direction and a 2mS pulse turns the arm fully in the other direction or anything in between, total movement is about 90 degrees. The servo expects these pulses approx every 20mS.
The length of this pulse is determined by the resistance of R3 + R4, 8K gives full movement one way and 20K gives full movement the other way.

Steve, in post #4 point1) you say to use a transistor to duplicate the action of the switch, but in post #6 you say to connect pin 3 to the junction of R3 and R4 using a diode, which is correct?
I would prefer to use a diode as it will make the circuit simpler, do you have a particular diode in mind?
I don't know why the output to the servo is through a transistor, could the current be too high for the 555? I'd be happy to leave it out if it still works.
Please feel free to post a diagram of any modifications as a picture says a thousand words.
Thanks Stew

 

Thanks Steve
The only specs I can find for the servo is that, at rest, there is a current drain of 80mA which presumably is the signal current plus any current used by the electronics inside the servo.
I'll test your circuit on a breadboard this afternoon and let you know how I get on.
thanks Stew

 

I set up the circuit below on a breadboard and everything works fine except for the time delay. When the water level is low the output of 358 goes low and the time delay immediately kicks in giving me minimum 10 seconds or until water reaches sensor again. I really wanted time for water to reach sensor + 10 seconds.
It's no big deal and I can live with it but it's not quite what I was after.
Apart from that the circuit works great and I am really happy.
If there is a simple answer let me know but don't be pulling your hair out.
I will be adding a couple more sensors in near future and I hope it's ok to call on your help and expertise if I get stuck.
1 to monitor water level in feed tank which will light an LED when water level low and also disable the pump in this circuit so that the pump doesn't run dry.
2 to monitor boiler pressure and operate another servo which will regulate the gas supply.
Thanks Stew

 

Hi Chris
Would that be a photo of the boat, the engine and boiler, or the water level sensor?
Are you a model maker, or do you just play with the big stuff?
Stew

 

I'll get you some pics tomorrow cdrive.

mk1 Colchester Bantam owner
http://www.lathes.co.uk/bantam/index.html

Stew