Can some one point me in the right direction?

[polish-Jack]

I have an idea for a very simple electronic project. I need a very small device that when the circuit is open the device is dead but then the circuit closes momentarily then a continuous, high pitch, tone is generated for three seconds and a small LED will light up for the same amount of time.

I participate in the sport of fencing (sword play). We have an electronic scoring system that leads a cable up the back of out protective clothing, down the sleeve and into a socket on the weapon. On the tip of the weapon there is a small button/momentary switch that when pressed closes a circuit in the scoring system, indicating a valid touch.

The problem with this is that there are only two such devices for the whole club. I want to make a little box that can be attached to the fencer, most likely at the wrist or somewhere along the arm. In this box there will be the circuit that I have described.

My knowledge of electronics is limited to construction of devices if I have a schematic. Design is out of my league. I have been told that such a circuit is a simple device that would use a 555 timer. Could someone provide me with a link to such a circuit, if it exists, so that I may try to adapt it to the weapon?

 

[John Sorensen]

What you're describing is called a "monostable." Try doing a search on Google for 555 monostable. I did, here's one:

**broken link removed**

j.

 

[polish-Jack]

Thanks for the information, but now I have a problem.

I just figurd out that the weapon itself is a closed system. When the button/trigger in the tip of the blade is pressed the wepaon becomes an open connection. This is what the scoring machine uses to detect a hit. Could I somehow convert the monostable circuit so that it is triggered on open circuit?

 

[evandude]

yes. at the input of the 555, you would want to wire the weapon switch between the input pin and +V, and put a resistor (say, 10k) between the input pin and ground.

this way, when the switch is closed, it pulls the input pin high (since the resistance is basically zero, it overwhelms the pull-down of the resistor), and when it is open, the resistor pulls the input pin low (which triggers the circuit)

 

[polish-Jack]

I understand what you said but I cannot picture it at the moment. I will figure it out. One question that I have is the current. The weapon has a hair thin insulated wire running up a grove that is in the blade and sealed with crazy glue. Will it be able to withstand the current flow if the circuit is constantly closed?

 

[evandude]

it shouldn't be a problem... you could put a resistor in series with the switch just to be on the safe side (must be much smaller than the pull-down, such as 1k if the pull-down is a 10k) but generally speaking the 555 will draw almost no current from the wire, so the only current that will go through it is through the 10k resistor to ground. with a 5 volt supply this is only 0.5 mA which would be safe for even an incredibly thin wire.

 

[polish-Jack]

yes. at the input of the 555, you would want to wire the weapon switch between the input pin and +V, and put a resistor (say, 10k) between the input pin and ground.

this way, when the switch is closed, it pulls the input pin high (since the resistance is basically zero, it overwhelms the pull-down of the resistor), and when it is open, the resistor pulls the input pin low (which triggers the circuit)

I can't figure this out. Can you describe it again using the diagram that I have linked to as a refrence?

https://www.eleinmec.com/figures/004_02.gif

 

[evandude]

here, just replace the 10k resistor and switch in that diagram with those in this diagram:

 

[polish-Jack]

Hah! So simple... That just goes to show you, a picture is worth a thousand words.

Thank you very much for your help.

 

[Tony Stewart]

You may get false triggering from electrical noise, present from 60 cycle fields that might interfere, also contact bounce,,but since the switch is closed mostly, a small cap of 0.1 uF of confirmation of a direct hit may be necessary. while suppressing ESD noise possibly from fencing sword contact.


The 555 uses a res*cap value or RC product constant. You need several seconds according a simple formula T=1.1*R*C where Ra ought to be around 100k max. thus for 3 seconds, C must be 27 uF approx. perhaps more so get extra parts like 33 to 47uF rated for any voltage greater than Vbat with good quality low leakage caps.

4.5V is specified as the minimum battery voltage, but if lower, it means the timing may shorten, so it might work from 3 AA's certainly 9V , then use a piezo buzzer rated for 5V to Vbat.

The LED can be an ultrabright running at 10mA easily with a current limiting R determined by (Vbat-Vf )/R usually in the range of 470'ohms.

Since the active output is low to gnd = V-, both buzzer and LED + sides go to V+ while the negative polarity of both devices are switched ON by connection to ground for 3 seconds.

 

[spec]

The LED can be an ultrabright running at 10mA easily with a current limiting R determined by (Vbat-Vf )/R usually in the range of 470'ohms.

You could even use one of the high efficiency LEDS which are very bright at 2 mA, the Vishay intense red LEDs particularly so. https://www.vishay.com/docs/83343/tlle4401.pdf

I suggest that the OP would be greatly helped by a complete schematic for his project- as he says a picture is worth a thousand words.

spec

 

[JimB]

You just revived a twelve year old thread.

JimB

 

[spec]

You just revived a twelve year old thread.

JimB

 

[atferrari]

Given the current occurrences around the globe he is training in shooting from the heap, 20 m away of multiple targets.