555 timer question for water system controller

Thread Starter

Rickyegrant

Joined Dec 20, 2017
5
Hi I'm working on a green house water mister controller, I'm trying out a simple monostable 555 mode to pulse a short output of 5v when the trigger goes low when a reed magnetic switch is closed, this 5volts is to actuate a solenoid to close a water valve. The problem I have is that the reed switch in this project can remain closed for around 15 minutes which would keep the 5v output going all that time draining the battery I've tried the circuit with just an LED and when you press the switch and leave it on the LED stays on all the time which is not what I want. Can anyone suggest how I can just achieve a 1 second pulse of 5v even if the trigger is set to low for a period of time . The circuit runs on a 9 volt supply
 

AnalogKid

Joined Aug 1, 2013
8,256
The 555 monostable circuit is not a true monostable because it does not have positive feedback and gating. These combine to make the output pulse completely independent of the input pulse. Once an edge is detected, a true monostable times out even if the input stays in the triggered state, and doesn't make another output pulse until the input goes away and returns.

The standard way to cure a 555 is to add a capacitor in series with the trigger input as shown above by AH. This sometimes is called a differentiator, edge detector, or edge extractor.

Is the 555 output driving the solenoid directly or through some kind of driver?

Schematic ? ? ?

ak
 

Thread Starter

Rickyegrant

Joined Dec 20, 2017
5
Hi thanks for your responses I will try out what you suggest in the circuit diagram . I'm hoping the 555timer will operate the solenoid directly as iit operates using 5 to 9volts I haven't got a diagram yet but will get one done.

I'm building an electronic leaf that is lowered from a thin layer of mist sprayed then rises as the mist evaporates thus maintaining a constant environment for propagation .I'm using magnetic reed switches to avoid any interaction or resistance to the delicate movement of the leaf arm. When the leaf rises the reed switch operates the solenoid until the leaf lowers which is fine but once down the solenoid need a reverse pulse to switch off the water but the leaf can be down for 15 minutes or more depending on temperature and weather conditions. Thus I don't want the reverse pulse to stay on for 15 minutes .electronics is not my area I'm more of a mechanical engineer.
 

Thread Starter

Rickyegrant

Joined Dec 20, 2017
5
Across both terminals of the solenoid to only allow voltage to the solenoid and not allow the voltage to flow back in the this spike?
 

Alec_t

Joined Sep 17, 2013
10,727
Voltage doesn't flow: current does. The diode cathode (the 'bar' end of the diode symbol) connects to the +ve terminal of the solenoid. The diode anode (the 'triangle' end of the symbol) connects to the -ve terminal of the solenoid.
 

Thread Starter

Rickyegrant

Joined Dec 20, 2017
5
Many thanks the edge trimming works a treat with an LED just got to wire up the solenoid circuit and then jobs done this is a very good forum!
 
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