Project Description

this project is a Target Hit Indicator utilizing a piezo sensor to detect vibration, and an opamp and 555/556 timer. it is attached with velcro to the back of a steel target and will blink LEDs when the target is hit. it is useful for long range shooting where confirming a hit is difficult, or for when you do not have a spotter. it is designed to fit in the ubiquitous Altoids tin, and minimizes the vulnerable portion to just the LEDs.

Operation

operation is fairly simple. turn on the board and wait for the initial blinking to stop. set the sensitivity and blink duration. then give it a whack. for best results, stick the tin and the LEDs on the REAR of the target. but hey, you're the one with the gun, so i'm not going to tell you what to do.

the SENS potentiometer changes the sensitivity of the piezo, with higher sensitivity turned to the left. there is a point where the LEDs will blink due to too much sensitivity. turn the dial to the right a little bit until the LEDs stop blinking, and that is the threshold. this is probably due to a flaw in my circuit.

the DUR potentiometer is the duration of the blinking. turning to the left is the shortest blink time. if you turn it all the way to the right, it will stay blinking at some point (not recommended). there is a minimum blink duration set by R4. i tried to make it about 3 seconds minimum. if you want a shorter minimum, use a lower value resistor on R4.

Supplies

Board Files

Circuit and board files are available here... https://www.dropbox.com/s/0mp11g0i261hz0k/HitIndicator.v20-updated.zip

List of electrical parts required (the list is also on a google doc with links to where i got the exact part, and will be maintained)

1x 9V M+F
1x MEAS Piezo
1x LM741
1x NE556
1x 100k ohm TrimPot
1x 500k ohm TrimPot
1x Switch 8mm, Latching, 12V
5x LED, Red Superbright
1x 5.6M ohm resistor, 1%, 1/4W
2x 100K ohm resistor, 1%, 1/4W
1x 15K ohm resistor, 1%, 1/4W
1x 5K ohm resistor, 1%, 1/4W
5x 220 ohm resistor, 1%, 1/4W
1x Capacitor, electrolytic, 47uF 16V
1x Capacitor, electrolytic, 10uF 16V
1x Molex connector, M, 3 conn, right angle
1x Molex connector, F, 3 conn, right angle
3x Molex crimp pins *** keep in mind you save a lot by buying at least 10+ pins, don't just buy 3, or 9.
2x Stereo jack, 3.5mm *** the stereo jack is difficult. i bought these locally, and modified a part in EAGLE to suit them. they look like con-lumberg 1503_09 but one pin is misplaced, hence the large mill slot. your best bet is to find one online that has an EAGLE model and substitute yours for mine. you also do not need it, if you want to solder wire in the holes.

List of tools required

dremel cutoff wheel, or thin kerf saw
solder iron, solder (optionally, solder flux)
molex crimping tool
pliers, needle nose preferred
wire cutters
wire strippers
wire shrink tube
6mm and 8mm drill bit and drill (or best imperial equivalent)
gaffers tape, or heavy duck tape
1" adhesive backed, strong velcro

Additional pieces

A 9V battery, a 3.5mm stereo cable, sturdy rubber band, and an Altoids-size tin (interior dimensions 91x55x20mm)

Places to save money in the build

if you want to reduce the unit cost, you can eliminate a few parts by not using quick connectors, and soldering things directly to the circuit board.

eliminate the 3 pin molex connector by soldering the power switch wires to the board,
replace the fixed 9V battery connectors with a simple one with wires and solder the wires directly
replace the 2x stereo jacks and stereo cable with long wire soldered directly into the two boards (though this limits flexibility of placement of the LEDs)
use less LEDs. i'm sure 3x should work well enough, though i have not tested it.
instead of a 3 wire illuminated power switch, use any 2 wire, latching power switch of 10mm diameter, or less (using more than 8mm will require cutting a recess in the lid to allow the lid to close)

Optional Builds

if you are wondering what components like R1 and LED1 and the smaller 9V connector are for, when they clearly do not get used, good question. this project evolved from a single high power 3W or 5W LED connected to an off-board stereo jack and required a larger resistor. i will likely pull these pieces out in a later revision, or leave them in and document them better. for now, ignore R1, LED1 and the 2 pin molex 9V connector. the third pin on the piezo connector is for an alternate piezo sensor type that had different spacing, and is also not used.

Known issues

i'm sure the resistor for the LEDs is too high at 220ohm, and should be closer to 20ohm. if someone can confirm this and make a suggested replacement value, i can test it out.

Step 1: Cut Off the LED Sub-board

cut the LED sub board from the main board. there is a dotted line to follow. be careful not to cut into the main board, as the stereo jack traces are very close to the edge of the board. sand the resulting edges.

Step 2: Solder the Components

solder the parts to the boards. there are many tutorials to follow if you are new to soldering (that explain things better than i ever could). using some solder flux is really really advisable, even when using flux core solder.

the parts are labeled on the board. pay attention to the polarity of the capacitors, ICs, piezo sensor, and ESPECIALLY the battery connectors. there is no polarity protection in this circuit. when inserting or removing the battery, make sure the power switch is OFF. if using a 2 pin connector for the switch, solder it to BLK and RED and leave GRN empty.

on the LED sub-board, the LEDs go on one side, and the resistors and stereo jack go on the other side.

handy trick #1: for parts other than resistors and caps, fold over the pin or tab on the underside before you solder, rather than cutting it off afterwards. this leaves the part reusable if you later take it off the board.

handy trick #2: if you cannot easily find right angle 3 pin (or 2 pin) molex, you can convert a straight pin version if you push the pins through a millimeter (extending the solder side by shortening the connector side), then bend the pins down (try to make sure they do not pull out any further when bending them down).

handy trick #3: blue tack is useful for holding components in place for soldering, BUT it melts slightly when heated, so try not to put it on exposed metal that will be soldered. stick it to the body of the component for best results.

the order i follow for soldering the parts goes like this… main board first, then sub-board

9V battery terminals (one + and one -)
piezo sensor
switch terminal (with pins facing to the right)
trim pots (potentiometers VR1 and VR2 [mistakenly labeled VR3 in the photos])
opamp and 556 ICs (IC1 and IC2)
resistors and capacitors (R2 through R6, C1 and C2)
stereo jacks on both boards (JACK1 and JACK2)
sub-board LEDs and resistors (R7A through R7E, LED2A through LED2E)

Step 3: Test Your Circuit

test your circuit. you can power it on just by shorting the top two power pins with your pliers when you have a battery connected, and the stereo cable connecting the two boards if you see blinking LEDs, it is working.

Target Hit Indicator - Step #3
Download

Step 4: Prepare the Altoids Tin

the placement of the stereo jack hole is very important. the power switch hole has more leeway. i use calipers to scribe a line 12mm from the top edge of the tin bottom. this is the centreline for the holes.

from the front of the tin, a minimum of 18mm (EDIT: 19mm is better, this leaves more room between the board and edge of the tin), scribe a vertical line intersecting the horizontal line you scribed previously.

scribe a vertical line 20mm from the back of the tin, also intersecting the horizontal line. these will make the centre points for the two holes.

use a punch or large nail to make a small indent at the centre points as a drill guide.

i cut a small piece of wood to support the underneath of the tin when drilling, and clamp it in a vise. you can test fit the board when the stereo jack hole is drilled. make sure to clean up the drill holes with a file, and make sure any metal shavings from drilling are removed from the tin (you don't want to short out your new board with a metal chip).

Step 5: Wire the Power Switch

if you are using the illuminated power switch i use, you will need 3 wires and a resistor. soldering the switch can be tricky and requires somewhat delicate wire placement and soldering. you want to take care to not short the connectors on the switch. it is really hard to photograph the switch wiring, but the drawing will illustrate the correct place for the wires and resistor.

handy trick #4: use some solder flux and tin the wires and switch pins before trying to solder them together. tinning means, for example, to put a small amount of solder one some wires, just enough to keep them together while they pass through the hole, but not enough to block them from going through the hole. tinning will also make the joining of the two parts easier.

each switch may have different wiring, so i cannot help with all the options. the switch i use has 4 pins; 2 brass and 2 silver. the brass pins are the power, and silver are for the internal LED. you can ignore the LED if you want, and you can use a 2 pin molex. the third pin of the molex is the ground for the LED.

the switch type used is an SPST (i think). it has an in and an out pin, or a hot and cold pin, not really sure of the naming. i wire the RED to one brass pin (HOT), and the BLACK and one end of the resistor to the other brass pin (COLD). think of it as the power is coming in on RED, and out on BLACK, when the switch is ON. the current then also goes through the resistor to the LED and through the GREEN to ground. the weird thing about this particular switch is the LED will light when there is current on either of it's silver pins. i would expect an LED to act as a diode, and only allow current in one direction, but that isn't the case with this switch, either silver pin will supply the LED.

look for a good video to explain crimping, rather than have me explain it. strip the other ends and crimp the molex pins onto the wire, and insert them into the molex connector to test the switch before installing it in the tin. you can remove the molex connector after testing by carefully pushing in the retaining tabs and gently pulling on the wires to get them out of the connector.

Step 6: Install the Power Switch

make sure you remove the retaining nut and ensure the O-ring is on before putting the wires through the hole.

put the retaining nut over the connectors and thread onto the switch, then tighten the nut (you want it more than finger tight but not super ratcheted down, as the threads on the switch can strip)

slide the shrink tube over the wires, but don't shrink it yet

put the molex connector over the pins and plug it into the board for another test. you do not have to put the board in the case, yet for the test.

when you are happy it is working, shrink the tubing CAREFULLY. if you already have the tape in the tin, it may melt a bit while heating the shrink tube.

line the bottom of the tin with some heavy gaffers tape or duck tape (i prefer gaffers tape, but use whats ya got!). this protects the circuit from shorting out on the metal tin. i choose to line the tin rather than put the tape over the bottom of the circuit because the board is loosely mounted, and the high points on the bottom of the board will be hot spots for some rubbing that may occur. i have thought about cutting out a piece of milk jug to use as a liner, as it is thicker and still non-conductive, but never needed the extra protection.

NOTE: once you have the shrink tube heated in place, you cannot remove the switch from the tin without cutting off the shrink tube.

Step 7: Install the Board

leaving the board unplugged from the power switch, and without the 9V battery, slide the right side under the power switch and line up the stereo jack with its hole while pushing the board into the case. it is a tight fit, just be careful.

once in place, screw on the retaining nut on the stereo jack, and plug in the switch and 9V battery. plug in the stereo cable and power it on. if it is behaving different from your initial testing, then you may have a short, somewhere. possible places are sharp bits poking through the gaffers tape, or the power switch wires have started touching.

Step 8: Install on the Target

i cut three strips of velcro, a bit less than the width of the circuit board. i always keep them the same width apart when sticking it to the back of the tin and target, and i always use the same "side" of the velcro. this makes them interchangeable if you have multiple targets. the LED sub-board has a note on which side to apply the velcro, but it is always the same side as the LEDs. you can put the indicator anywhere on the target, as long as the LEDs are visible from the front. alternatively, you can put the indicator away from the target using a longer stereo cable.

that is it. leave questions in the comments and i'll try to answer them as best i can. i am already working on a new version that uses 2x AA batteries and low power ICs, but i don't think it will be much better than this one. a 9V battery will last a day at least... longer if you excel at missing the target like i do!