[GreenKeys] tuning fork frequency?

[gil at baudot.net]

Hi:

Well, you can also parallel the led with pretty-much-any-old diode,
across the led in the opposite polarity.  In the forward (led)
direction, the led has its forward drop of about 1.4V, with the rest
across the resisitor, and in the opposite direction, there will be about
-0.7V across it (and the rest across the resisitor).  The diode sees a
max reverse voltage of -1.4V, and the LED sees a max reverse voltage of
-0.7V.  An old trick.

But Ken is correct that this will only give you 60-Hz flashes, while
using a bridge will not only prevent blowing the led from reverse
voltage, but will give you 120Hz flashes.

gil

-------- Original Message --------
Subject: Re: [GreenKeys] tuning fork frequency?
From: Ken Schwieker <ksweek at mindspring.com>
Date: Sun, March 16, 2008 8:10 am
To: greenkeys at mailman.qth.net

There is a problem with the LED/resistor strobe circuit in that a 
LED's reverse max voltage spec (usually around 5 volts) precludes 
having the 140 volts on the non conducting half of the cycle getting 
to the led. A blocking diode should work to keep this from happening. 
Another better option would be to use a bridge rectifier which would 
in addition to solving the reverse voltage problem, give 120 blinks a 
second verses the 60 with a blocking diode. The blinks from a LED in 
these circuits will be rather drawn out which will result in a 
smeared target view. Since any useful strobe effect will work this 
probably makes no difference.

Ken S



 >Thanks for the help with the Teletype tuning fork question.
 >
 >I believe that 120VPS is the correct number for a Model 28, the 
87.6VPS number is for a Model 15.
 >
 >The LED/resistor wired across the AC line is an ingenious idea.
 >
 >Dave Ross N7EPI

_______________________________________________
GreenKeys mailing list
GreenKeys at mailman.qth.net
http://mailman.qth.net/mailman/listinfo/greenkeys