555 voltage output low

Thread Starter

KLillie

Joined May 31, 2014
137
I was trying to oscillate a 555@ 1MHz. My input voltage was about 8 volts, but my dc peak was around 1 volt. Later I saw that your run of the mill 555 doesn't like to be run much above 350 kHz. Could the frequency have been the cause?
 

KMoffett

Joined Dec 19, 2007
2,918
"Could the frequency have been the cause?" The 555 won't operate properly above about 350KHz. You are trying to run it at 1000KHz. Yes, that is the logical conclusion? :)

Ken
 

ScottWang

Joined Aug 23, 2012
7,397
What is the oscillator used for and does it need to adjustable?
If the oscillator is not adjustable then you could buy a 10Mhz crystal oscillator module and divide it by 5 and 2 to get a precisely 1Mhz has 50/50 duty cycle.
 

Alec_t

Joined Sep 17, 2013
14,263
If the oscillator needs to drive only a very light load (a few mA) you could make one simply from a CMOS Schmitt gate such as a CD40106.
 

ian field

Joined Oct 27, 2012
6,536
"Could the frequency have been the cause?" The 555 won't operate properly above about 350KHz. You are trying to run it at 1000KHz. Yes, that is the logical conclusion? :)

Ken
Apparently (allegedly) the CMOS 555 can do 2MHz, I was told the limit for a bipolar 555 was 100kHz but I'm sure I've had more than that from one - 350kHz isn't that far out if you're not too fussy about squarewaves actually being square.
 

ScottWang

Joined Aug 23, 2012
7,397
Apparently (allegedly) the CMOS 555 can do 2MHz, I was told the limit for a bipolar 555 was 100kHz but I'm sure I've had more than that from one - 350kHz isn't that far out if you're not too fussy about squarewaves actually being square.
The datasheet mentioned that the mic1555 can be oscillate reach to 5Mhz when it using RT = 1k, CT = 47pF, Vs = 8V, but I don't have this chip in hand, and it's quite expensive around US$1.23/ 1pcs in Taiwan online store.
 

ian field

Joined Oct 27, 2012
6,536
The datasheet mentioned that the mic1555 can be oscillate reach to 5Mhz when it using RT = 1k, CT = 47pF, Vs = 8V, but I don't have this chip in hand, and it's quite expensive around US$1.23/ 1pcs in Taiwan online store.
Past experience has made me wary of some datasheet claims.

Years ago I bought a kit to build a Colour TV pattern generator, both timebases came from 555s - the 50Hz vertical timebase was no problem, but the 15.625 kHz horizontal generator was all over the place.

The 555 is certainly capable of more than the PAL horizontal frequency, but its getting into the realms of stray reactances around the PCB getting significant compared to the specified values.

After about a week of experimenting with ways to make the timebase stable, I just gave up and built a TTL timebase generator dividing down from 10MHz.
 

ScottWang

Joined Aug 23, 2012
7,397
Past experience has made me wary of some datasheet claims.

Years ago I bought a kit to build a Colour TV pattern generator, both timebases came from 555s - the 50Hz vertical timebase was no problem, but the 15.625 kHz horizontal generator was all over the place.

The 555 is certainly capable of more than the PAL horizontal frequency, but its getting into the realms of stray reactances around the PCB getting significant compared to the specified values.

After about a week of experimenting with ways to make the timebase stable, I just gave up and built a TTL timebase generator dividing down from 10MHz.
Using a 40Mhz crystal oscillator module, and get some counter IC as 74HC390, 74HC393, 74HC4040, and some dip switches, some divided by 2, some divided by 5, some divided by 10 then you can get a precisely oscillator.

I used some crystal oscillator modules as 40Mhz, 32Mhz, 24Mhz and the above method to made a different kinds of frequency precisely oscillator.

I also made a NE555 oscillator as below.
 
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