low power solenoid for an embedded circuit?

[rashidme]

Hello,

I am working on a project where a door lock will be controlled from a smart phone. The mode of communication is low power bluetooth (BT 4.0). The battery constrain is 3V (CR2032). The microcontroller (nanowatt) will usually be sleeping except when a connection is initiated from the phone only then will it wake up and process the request and go back to sleep.


my questions?
- should I use solenoid or a low power dc/stepper motor to lock the door and unlock it?
- my main concern is power, what factors should I be looking at?
- any suggestions on low power pic microcontrollers?
- I would be using an led for low battery status, would that affect power?

I am a software developer and have little experience with hardware but I am really interested in designing hardware, so please help me out here

 

[ADWSystems]

Door locks come in two typical styles. The first allows simply releases the door to allow you entry, usually a one time admittance per release. The second unlocks the door lock itself (like a car door), most of these are actually a motor. With a motor, two diodes and two limit switches you set the motor up to turn off when the limit switch is tripped thereby preventing it from going into a stall and gulping current.



That would not require a complicated stepper motor driver.

Unfortunately, any of the three options will most likely drain a CR2032 in a few actuations. They typically have a 240mA-Hr capacity. The 3V motors I quickly looked at draw 100mA. Granted it will only fire for a few seconds, but you cannot draw all of the 240mA-Hr from the battery for just the actuator. It must also supply the micro and other circuitry. All of the stuff has a minimum voltage requirement, and where it is starting at 3V, there isn't a lot of headroom left for the battery voltage to droop before the circuit shutdowns for low voltage.

Is there any way of getting source with more mA-Hr capacity or possibly a second source that could drive or recharge the power source for the unlatch mechanism, perhaps solar recharge?

FYI. Even AA batteries offer 2100mAHr capacity. Almost 10x the CR2032. All Electronics even has a 3.6V AA 2400mAHr battery that might work really well. https://www.allelectronics.com/make-a-store/item/LBAT-124/3.6V-AA-SIZE-BATTERY-W/LEADS/1.html

 

[rashidme]

Door locks come in two typical styles. The first allows simply releases the door to allow you entry, usually a one time admittance per release. The second unlocks the door lock itself (like a car door), most of these are actually a motor. With a motor, two diodes and two limit switches you set the motor up to turn off when the limit switch is tripped thereby preventing it from going into a stall and gulping current.

View attachment 75848




That would not require a complicated stepper motor driver.

Unfortunately, any of the three options will most likely drain a CR2032 in a few actuations. They typically have a 240mA-Hr capacity. The 3V motors I quickly looked at draw 100mA. Granted it will only fire for a few seconds, but you cannot draw all of the 240mA-Hr from the battery for just the actuator. It must also supply the micro and other circuitry. All of the stuff has a minimum voltage requirement, and where it is starting at 3V, there isn't a lot of headroom left for the battery voltage to droop before the circuit shutdowns for low voltage.

Is there any way of getting source with more mA-Hr capacity or possibly a second source that could drive or recharge the power source for the unlatch mechanism, perhaps solar recharge?

FYI. Even AA batteries offer 2100mAHr capacity. Almost 10x the CR2032. All Electronics even has a 3.6V AA 2400mAHr battery that might work really well. https://www.allelectronics.com/make-a-store/item/LBAT-124/3.6V-AA-SIZE-BATTERY-W/LEADS/1.html

Thank you for the prompt reply. Is it possible to use a solenoid or a magnet thus eliminating motor at all? Or could I go with two CR2032 batteries in series? The thing is that lock must be portable which is where the constraints are coming from.

 

[ADWSystems]

Motor or solenoid are both electro-magnetic and thus have the same drawbacks (ie., require lots of compared compared to the rest of the circuit). Schlage offers a door lock, albeit isn't portable, but it runs from a 9V battery. You will definitely be better off with 2 CR2032 over just 1. The question will be series for 6V or parallel for 480mAHr. Do you have micro et al located that can run under 3V? I have heard but not seen of a 2.7V low voltage series but have tried to find any. We all know there is 3.3V chips available which would require the batteries in series.

 

[rashidme]

Motor or solenoid are both electro-magnetic and thus have the same drawbacks (ie., require lots of compared compared to the rest of the circuit). Schlage offers a door lock, albeit isn't portable, but it runs from a 9V battery. You will definitely be better off with 2 CR2032 over just 1. The question will be series for 6V or parallel for 480mAHr. Do you have micro et al located that can run under 3V? I have heard but not seen of a 2.7V low voltage series but have tried to find any. We all know there is 3.3V chips available which would require the batteries in series.

Just ran into this lock https://www.masterlock.com/products/product_details/1500eDBX how do they power this from a coin cell? There must be a mechanism to lock/unlock the latch?

 

[ADWSystems]

Last 5 years powered by a CR2032. Hmm? I would be highly skeptical if I could unlock it once a day and have the battery last 5 years. On the other hand, I wouldn't doubt you could lock it on day 0 then unlock it 5 years later.

 

[ccurtis]

I suspect a very low power motor or solenoid is used to push/pull/move a small, lightweight pin that engages/disengages the latch lever from the locking arm. Once the pin is engaged, the user provides the force required to move the locking arm. When the pin is disengaged, the operating latch lever can't do anything.

 

[rashidme]

I suspect a very low power motor or solenoid is used to push/pull/move a small, lightweight pin that engages/disengages the latch lever from the locking arm. Once the pin is engaged, the user provides the force required to move the locking arm. When the pin is disengaged, the operating latch lever can't do anything.

yes that is my point what kind of motor are they using so that it will last that long. I would like to shoot for such a motor.

 

[alec_t]

Something like this perhaps? All the gearing-down is done for you. Simple variable-width pulse controls it.
https://www.hobbyking.com/hobbyking..._5g_3v_.html?gclid=CKqE3L25w7kCFXHMtAodI3kAAw
Alternatively the vibration motor from a dead mobile phone could be a candidate.

 

[rashidme]

Something like this perhaps? All the gearing-down is done for you. Simple variable-width pulse controls it.
https://www.hobbyking.com/hobbyking..._5g_3v_.html?gclid=CKqE3L25w7kCFXHMtAodI3kAAw
Alternatively the vibration motor from a dead mobile phone could be a candidate.

that looks really nice. can be powered from 3V and has 1.8kg torque!! sweet. I didn't even think of the vibration motor. Thank you so much. They don't specify the current consumption though, unless I am blind.