[Noisebridge-discuss] charging a cap with a xtal?

[macegr]

That circuit will work, though if you want the voltage to fall when the oscillator is turned off, then you'd need a drain resistor too.

Please keep in mind that if this is a simple square wave oscillator that doesn't do anything else interesting, it'll charge the capacitor half as fast as a direct connection to VCC (minus diode drop for whatever diode you select). By changing the RC values you could accomplish the exact same thing with a piece of wire.


On Sep 12, 2013, at 12:38 PM, Johny Radio <johnyradio at gmail.com> wrote:

> On 9/12/2013 10:38 AM, macegr wrote:
>> 
>> Must you actually charge it with the oscillator, or are you simply attempting to generate a time-dependent voltage on the capacitor?
> 
> must actually charge it with the crystal osc, because our challenge is to make something with the parts given us by Instructables.com.
> 
>> If you're using a TTL oscillator module, it might be outputting a 5V square wave. This would allow you to charge a peak-detector circuit (diode inline to capacitor connected to ground, optional drain resistor) to within maybe 0.6V below VCC. This will be harder if you're trying to connect directly to an oscillator circuit without an output buffer.
> 
> ok, i can add a transistor buffer to the xtal output if necessary. I'm assuming the resistor will determine the charge time of the cap. Is this correct:
> 
>                                                 OSC--
>                                                          |
>                                                          |
>                                                       Diode
>                                                          |
>                                                          |
>                                                       Capacitor
>                                                          |
>                                                          |
>                                                       Resistor
>                                                          |
>                                                          |
>                                                        GND
> 
> 
> Johny Radio
>  
> 

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