Re: Sensing switch closure with & without power applied?
From: Rodney Josey (rjosDIESPAM_at_dodo.com.au)
Date: 01/09/05
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Date: Sun, 09 Jan 2005 19:51:38 +1000
On Fri, 7 Jan 2005 16:02:26 -0800, DaveC <me@privacy.net> wrote:
>Switch closure needs to be monitored. On one side of the switch is mains
>voltage (120vac); on the other, the load.
>
>Simple enough to monitor the switch's status: when mains voltage appears on
>the load side, it's closed.
>
>But how to monitor this switch during a power failure? I'd like to be able to
>report to an alarm sensor's input (simple current loop) when this switch
>closes, whether mains power is applied to the switch, or power has failed.
>
>When power fails, I could put a simple battery and coil of a l.v. relay
>across the switch. But when mains voltage reappears, removing the battery
>circuit in time becomes a bit risky.
>
>How best to accomplish this task? Ideas?
>
>Thanks,
------------------------------------------------------------------------------------
Hi Dave,
I think I may have come up with a circuit to meet your requirements...:).
Unfortunately it is a bit too much to acurately lay it out using AACircuit (www.tech-chat.de) which
is what I generally use for posting to this N.G. - so I've split it into smaller sections.
If I'm reading what you say in your posts right - your system comprises a 120VAC submersible pump
(in a well) controlled by a float switch (which is in series with the pump) - AND - you want to be
able to monitor the state of the float switch - AND - set an alarm input depending on whether the
switch is closed or open - IRRESPECTIVE of whether the 120VAC power is on or off ????
QUESTION: what is the max current draw of the pump motor ?
If I'm correct with my summizing, then the circuit I propose would certainly perform the task plus
give some added FLEXIBILITY to how you can use it. The circuit uses around 35-40 components
mounted on a PCB and along with a 4 x AA NI-MH (or NICAD) batteries (1.5V ea @ 2300mAh = 6V DC)
should fit into a 8"x 6"x 3" plastic enclosure case and cost approx $75.00 or less (depending on
where you purchase the components).
The circuit concept is: If the in-well pump is say 120VAC @ 10A then the minimum 'operating' voltage
before the pump motor stalls is probably around 60VAC @ 5-6A - so at lower AC the motor doesn't turn
- however - the motor windings will still pass low voltage / low current AC (the motor just wont
turn) - THUS - this effect can be used to easily determine the "state" of the in-line float switch -
e.g.. if the low voltage / low current AC flows thru the motor - then the float switch is closed -
if not - the float switch is open - VOILA - a method to determine the float switch "state". NOTE:
this will work whether the normal AC to the pump motor is on - or - off (actually 'off' manually -
or - when mains power fails due to a blackout or low power condition - brownout). NOTE: AC voltage
is used instead of DC because AC motors may have internal 'capacitive' coupling as well as inductive
- DC is generally 'blocked' by such coupling methods - AC will pass thru.
A transformer-less AC-to-DC 12V DC power supply across the Mains active and neutral lines drives 2 x
10A DPDT 12vdc coil relays - 1 relay in the mains active line - 1 in the mains neutral line
(ISOLATES the 'outside' well mains wires from the 'inside' well mains wires). A 3rd low power SPDT
relay 12vDC coil is also operated by this power supply and is used to turn on / off the battery
power to the 'control' circuit. You'll need to work out the component values to suit.
---------------------------------------------------------------------------------------------
MAINS RELAY SUPPLY:
Active (outside .------.
Mains Well) RLY1|--o\ |
120V o------+-------+-----------------------------------------|--o o-|--+---------o 'ACTIVE'
AC | | | | | (inside
+ .-. +--------|--o\ | | Well)
--- | | R1 | _____|_-o o-|--+
C1 --- | | | | | )| |
+ '-' | | _|_)| |
| | | | |'------'
| | | | |
| .-. +---|---|--------------------o Txfr1
| | | R2 | |
| | | | +---+
| '-' | |
| | +--|<---+
+-------+ | D2 |
| | |
.-. .-. |
| | R3 | |R4 |
| | | | |
'-' '-' |
+ | |
+----+----+ | +----o Gnd To Relay 3
| ~ | | |
- V | |
^ - +-------+-------+----Xa-----+--+----|----o +12V To Relay 3
DC | BR1 | | |+ + | |
+-----+ +---+ --- z | |
| | | DC --- A 12V .-. |
| V - | C2 + Zener R5| | |
| - ^ |470uF | | | |
| | ~ | |25V | '-' |
| +----+----+ | | | |
| | | | | |
+----------|----------------+-------+-Xb-Xc-+------+----+
| | |
| +-->|--+ .------.
120V | | D3 | |--o\ |
AC o---------------+-----------------------------------------|--o o-----+--------o 'NEUTRAL
Neutral | | | | | (inside
Mains | +-|-|--o\ | | Well)
(outside | | |_|_-o o-|---+
Well) | | | )| |
|____|___|_)| |RLY2 +----o Isense-B
| '------' |
| |
+------------------+
NOTE: Xa, Xb, Xc = OPTION
(created by AACircuit v1.28.4 beta 13/12/04 www.tech-chat.de)
----------------------------------------------------------------------------------------------------
Whilst ever the mains is on - the relays are operated via the 12vDC - letting the mains AC thru to
the float switch & pump in the well. If the mains is turned off (manually) - or - fails (blackout),
the relays de-activate. The 3rd relay turns on the 'control' circuit - when power fails (or is
turned off manually).
The 'control' circuit (shown below) is formed as a battery operated low-voltage (5v DC) DC-to-AC
converter providing around 15-18VAC @ 100-150mA which is coupled to the 'inside' well mains wires by
a small low current step-up transformer via one pole of the each of the "isolation" relays. An
PICAXE-08M microcontroller is at the heart of the converter / monitor - 2 outputs drive MTP3055E
MOSFETS which switch 0v to one side of upper & lower windings (secondary) of the transformer - +5vdc
is fed to the centre tap of the windings. The output winding (primary) steps up the generated
voltage to around 15-18VAC @ approx 100mA. This generated AC voltage now flows - thru the 'active'
line relay - to the float switch (if closed) - thru the pump motor windings - to the 'neutral' line
relay - thru a 'current' detector - back to the other side of the transformer output winding.
-----------------------------------------------------------------------------------------------
CONTROL CIRCUIT:
+--------+ +--------+-------|<-----o +5V
| | | | Dx
| | --- |
Txfr-A | +--| --- 100n |
o | Q2 | | |
| | +->|| ___ === |
| +--+ +--|+---|___|--+ 0V |
+--. | | Ra | | U1
)|.---+ | | .------o-----.
TXFR )|(______o +5V === | | +ve | ___ Rc
)|( 0V +------oP4 P0o----|___|------o 'Alarm O/P'
)|'---+ | | ___
+--' | +----------oP2 P1o----|___|---+
| +-----+ | | | Rd |
| | | +----oP3 Sio--+ |
o | | | | 0V | | .------.
Txfr-B +--| | | '------o-----' .-. | | >
(Isense-A) Q1 | | | | | | | | < Opto_2
+->|| ___ | | | | |Re | V > Led
+--|+----|___|---+ | === '-' | - < (Option)
| Rb | 0V | | | >
| | | '------
=== o === |
0V 'sense' 0V |
===
0V
Q1,Q2 = MTP3055E FET
U1 = PICAXE-08M
Dx = IN4004
Resistors = to suit
(created by AACircuit v1.28.4 beta 13/12/04 www.tech-chat.de)
-----------------------------------------------------------------------------------------------------
The 'current' detector is shown below - when sufficient current flows thru the resistor Rx, the
optoisolator's led turns on. The opto's output transistor turns on - pulling the 'sense' line low.
THUS: if the float switch is closed - the 'sense' line is low; if the float switch is open - the
'sense' line is high.
------------------------------------------------------------------------------------------------
CURRENT SENSE DETECTOR CIRCUIT:
Txfr-B (Isense-A)
o
|
|
|~ +----------o +5V
+-+-+-------|--------+ |
+ | | | +
- V | .-. .-.
^ - .-.Rx | |47r | |1K0
Diodes | | | | | | | |
- +---+ + | | '-' '-'
| | '-' + +
V - | | +-----+----------o Sense
- ^ | .-+-------+--.
| | | | V |/ |
+-+-+-------+ | - -| |
|~ | | | |> | OPTO_1
| | | | + |
| | '-+-------|--'
| | | |
| +--------+ +----------------o 0V
|
o (Isense-B)
AC Neutral
via Relay
(created by AACircuit v1.28.4 beta 13/12/04 www.tech-chat.de)
------------------------------------------------------------------------------------------------
The 'sense' line is monitored by 'IN3' pin of the PICAXE-08M chip. A 3rd 'output' of the PICAXE-08M
is then used as the "alarm" drive out either via a 2nd OPTO - or - via a transistor....
------------------------------------------------------------------------------------------------
BATTERY SUPPLY:
.------. Dv
+ve | /o-|---->|-------o +5V
o----------------|--o o-| (control)
| ____|__ |
| | | )| | Rly3
--- | _|__)| |
Bat - | | '------'
--- | |
6V - | +------+
2300mAh | | |
NI-MH | | |
o-------------------------------------o 0V
-ve | |
| |
| |
+----|<---+
| Ds |
| |
| |
o ===
+12V Gnd
(Relays)
(created by AACircuit v1.28.4 beta 13/12/04 www.tech-chat.de)
------------------------------------------------------------------------------------------------
FLEXIBILITY:
The "flexibility" I mentioned earlier: using the PICAXE micro gives you the option (with a few extra
components in the "mains" relay power supply circuit (1 x resistor, 1 x opto, 1 x MTP3055E FET)) -
to 'delay' the turn-on of the "mains" to the float switch / pump motor AFTER the float switch is
detected as 'closed'. Example: if the float switch is detected as 'open' - the water level in the
well is below the "minimum" pump activate level. When the float switch is detected as 'closed',
you may want to delay the pump turn-on to allow time for the water level to rise sufficiently to
provide a suitable pump 'head' flow. A 'delay' sequence can be programmed in the PICAXE micro to
achieve this. An additional option: extremely low power consumption from the batteries by using the
PICAXE 'sleep' function :- testing of the 'sense' loop could be done once every 1-2 minutes (should
take about 3-5 seconds for a sampling) - in between each 'test' the micro could be placed in sleep
mode - conserving power. This could give several months of use from a set of batteries.
-------------------------------------------------------------------------------------------------
OPTION: Insert the components below into the "MAINS RELAY SUPPLY CIRCUIT" above where indicated.
'Xa'
o
|
|
|
.-.
| |Rm
| |
'-' +----o 'Xc'
| |
+-------+ |
.-------. | |--+
> | | | ||
Opto_2 < |/ | | ||<-+ Q3
O/P > -| | +--+|--+
< |> | |
(Option) > | | |
'-------' |
| |
'Xb'o---------+--------------+
(created by AACircuit v1.28.4 beta 13/12/04 www.tech-chat.de)
-----------------------------------------------------------------------------------------------
The PICAXE series of chips are very easy to program and are very low in cost - MUCH BANG for little
BUCKS - in such small packages.
I hope this extra long post is helpful ??
Best of luck with your project.
Oz-Rod
- Next message: Lady Chatterly: "Re: OT: Safe Riddles"
- Previous message: Kevin Aylward: "Re: OT: Let's Take the Religion out of Christmas"
- In reply to:(deleted message) DaveC: "Sensing switch closure with & without power applied?"
- Next in thread: Rich Grise: "Re: Sensing switch closure with & without power applied?"
- Messages sorted by: [ date ] [ thread ]
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