Please suggest a best and reliable switch circuit

On 18 Aug 2005 22:06:42 -0700, mingyan snipped-for-privacy@yahoo.com.hk wroth:

If I were doing it, I would make two oscillators. One at each frequency. That could be done with one 74HC04UB. Then I'd switch the oscillators into the 14060 with a quad NAND gate wired as a data selector. How to get the selection level, pushbutton or simple SPDT switch, is left as an exercise for the ambitious student.

Jim

Thank you so much for your advise, cause other chips are CMOS, any suggestion other than 74HC04UB? How about using two MC14060, then using a flip-flop to switch the clock to next stage. Which solution would perform better?

Sorry for my mistake. 7404 actually is a CMOS .

I am totally convinced not to use realy. Thank you so much, I am searching information to build a gate for the purpose of press the button once, it turn on one crystal, If I press it for over 5 seconds, it would turn on other crystal. Any advise will be highly appreciated.

You actually want to switch crystals ?

Wrong approach. It takes time ( millisecs ) for oscillators to start up so you won't get a nice transition.

Just run the crystals is individual oscillators round an inverter. Intel does a nice app note on crystal oscillators btw.

The oscillator outputs can then be gated for use as the counter clock. NO relays.

Graham

Conceptually, this is what you want to do, if I understand it: Press the pushbutton and immediately set the flipflop to high to select XTAL1, but if you hold the pushbutton down for 5 seconds, the FF is reset and at that time XTAL1 is deselected and XTAL2 is selected. In other words, you are not looking for a 5 second time where neither XTAL1 nor XTAL2 is selected. If I understood it correctly, the circuit below should work for you.

---- ------ +---[Relay]--Vcc PB----+----------|NAND|----|SET | /c | | | | FF |---| | | | | | \\e [R1] +-| | +-|Reset | | | | ---- | ------ Gnd +---+--[Zd]--+---------+ | | | [R2] [C1] [R3] | | | +---+--------+---Gnd

When you press PB the NAND output goes high and the FF is set so its output goes high. When C1 charges through R1 to the zener voltage, the NAND goes low bringing the set low. The reset goes high, resetting the FF, so the FF output goes low. R1 and C1 set the time it takes to reach the Zener voltage. R3 ensures the second NAND pin & the reset pin are not "floating" when the charge is below the zener voltage. R2 discharges C1 when the pusbutton is released.

You can use the output from the FF to select your XTAL - high selects XTAL1 and low selects XTAL2. A diode or FET or relay switch circuit can be used for that.

Ed

I'd appreciate your advice. Please frogive me for being lack of knowledge in electronic. Is ther any CMOS intergrated with both NAND gate and Flip-Flop. My board space is very limited. I am thinking if it possible to use two single chip oscillators then feed their output to the data selector. Thanks anyhow.

Wouldn't a J-K flop do what you want?

...Jim Thompson

Sure - you could use a CD4011 quad nand. Cross connect two of the nands and you have a flipflop.

Ed

That link provides a very useful 4011 application that i can make use of it.

Hi Ed,

I connect the circuit as you descripted. Using Nand gate to trigger the FF. The FF output is high once it is powered on. Not press the buttom. However, it meets the requirement of press for 5 seconds FF Q trun low.

When Q is low, does it go high when you press the button for

Perhaps I misunderstood. You have two xtals: a 3.xxx and a 4.xxx. You want to select the 3.xxx with a short press on the button, and the 4.xxx with a long press. The FF has to power on in one state or the other.

Do you really need 3 states?

OR do you need it to power up with the 3.xxx selected

OR can you never get Q to go high, once it has gone low?

Ed

Meant to ask you to post your circuit in my earlier reply. Ed

ehsjt, Thank you so much for your help.

Actually, I need 3 stage:

1. Press the button once(trigger it on), it select 3.XXMHz and turn on a red LED.
2. Press the same button for 5 seconds, remains on and select 4.XXXMHz. a,d turn on yellow LED.
3. Press the button again(trigger), it turns off the clocks feed to the op amp and turn off LED.

Actually, there is a MC4013 in the existing circuit handling the triggering to select the 3.XXXMHz oscillator clock feed through to the op amp through another MC4013. Therefore, I can make use of the existing 4013 to trigger the newly added oscillator selector. The MC4013 can also trun off the both oscillator and LEDs

I can use the spare MC3346 transistor array in the existing circiot to drive the LEDs on and off..

Does it work??

Thanks

I just realized that you may not have changed the flip flop from cross connected NANDs to the JK type Jim Thompson recommended. Use a 4027 chip for the flip flop. Ed

Ok that changes things. Use a 4017 decade counter. Conceptually: Run the pushbutton through a debounce to put a high into the clock input. When the counter is reset, 0 is active (high). (If you want, you could put a LED on it to indicate standby mode.) Press once, and 1 becomes active. Press the second time, and 2 becomes active. Press the third time, and 3 becomes active. Tie 3 to the reset pin; tie the reset pin to 0V through 10K. When 3 becomes active, the counter resets to zero. Use the 1 and 2 outputs to control xtal selection & light the LEDS.

You get 3 states - counter 0 or 1 or 2. You can use

Ed