Lost my post again. Either 12V or 24 V solenoid will work. Just need a resistor & 12 V zener diode or 7812 VR for low V part of ckt. Keep LED, good for testing. I believe that the Jameco # 1956784, $ 3.25 or # 336314 $ 1.59 speaker would make a good seismometer. I like the 5 cm but the 2 cm has advantaje of having a case which with care back can be removed to add a weight.
What part of the world are you in? I have a pot full of the 2 cm sensors available.

"Either 12V or 24 V solenoid will work. Just need a resistor & 12 V zener diode or 7812 VR for low V part of ckt."
You are talking about if I chose to go with a 12V solenoid right?

"I like the 5 cm but the 2 cm has advantage of having a case which with care back can be removed to add a weight."
Add weight? what do you mean?

I am on the East Coast, Virginia.

So I will buy both of these, how do I make them trigger that solenoid though? That is my next problem

 

You need a low V supply for sensor ckt no matter what solenoid you select, I would go for 12 V for reliable FET operation. Drive for solenoid depends on V & resistance. A 12 V solenoid, think I would go with capacitor discharge operating from 24 V. I.m short on time today, but will make a comparison fron naked 2 cm sensor & same with mas -- made one to try.

 

Comparison of sensors, target 8.5" X 11" sheet of paper supported horizontally with rubber bands, sensor taped 3" from long end, centered, target 3" from other end. Shot is BB dropped from 12" on to target.
Naked 20 cm disc = .5V, 20 cm with weight = .6V, speaker with 3 penneys =.05V, 64 ohm speaker with 5 gm weight =.01 V.
Next test, same target virtical with naked 20 cm disc suspended from bottom center. Projectile, BB fired from 5 ft with blow pipe..
Receiver, 555, Vcc 5V, Pin 2 biased via 3.5 meg pot, sensor connected between ground & pin 2, 555 setup as 2 sec one shot.
Can use 2 resistors, & pot instead of origional3.5 M pot. Bias on pin 2 could be cranked up to 3 V before hits became eratic. There are many variations for output, transistor, or logic level FET or ?, depending on selected solenoid & back pressure on plunger.
Not shown, voltage reg. & caps.
Sensor got lost when changing R 1-2-3; it connects from ground to pin 2. On test red lead to ground for neg leading edge pulse. Also LED & resistor p 3 to gnd. for monitoring.

 

Tried with sucess a 20 mm muRata piezeo speaker attached to target with small C clip. Like JameCo 336314 but with out ears. With the ears, could clamp to target with paper clips?
If target is hard clamped to cross member vibration near clamping point ( head) will be supressed.