# Variable Hertz

#### colum

Jul 25, 2013
174
I'm trying to clear up old projects and I need help with this last one.
I want to generate an AC signal at variable 350 t0 450 HZ at 115 Volts and about 10 Watts max. a square or Sine or Triangle wave would do and it would be nice if it was based on a 555 since they are easily available
and it would need to have the Neg and Pos parts of the Wave

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,865
In your previous thread (now closed) on this subject, you stated you would feed a low-voltage signal into a transformer and step that up to the 115 V AC required to operate the solenoid in the reed frequency meter. So far, so good. A 6.3 V center-tapped filament transformer with a 115 V AC primary should do nicely. Although most of these are designed for 50 Hz or 60 Hz primary excitation, they will have enough iron to serve at 350 to 450 Hz.

If you use a 555 to make a variable frequency oscillator, you should use the output to clock a D-type flip-flop to create complementary square wave output at half the 555 frequency on the Q and /Q outputs of the flip-flop. So the 555 would oscillate between 700 and 900 Hz. Use the two flip-flop output signals to drive a pair of transistors (2N7000 MOSFETs are easy, but NPN power transistors will also work) connected as switches between the two secondary terminals of the transformer and circuit common. Connect the center tap of the secondary to a +5 V DC supply. You should insert a current-limiting resistor in series between the center-tap and the +5 V DC supply. Use diodes across the transistor switches to protect them from "flyback" voltage generated by the on/off switching. The square waves from the flip-flop will alternately turn the transistors on and produce a high voltage in the transformer primary. You can play around with how much current gets switched by appropriate sizing of the current-limiting resistor in series with the center-tap and the +5 V DC supply. You can even try adding a resonating capacitor to the load side (the primary of the filament transformer) to see what effect this has.

This isn't "rocket science" and should be well within your capabilities. BTW, pin-outs on a 741 op-amp were shown in your first post on this thread. Most folks would just Google "741 datasheet" to get pin-outs. The pin-out sometimes varies with the particular package, which you failed to specify. Do you need help building a 555 variable-frequency oscillator or choosing a flip-flop?

#### colum

Jul 25, 2013
174
In your previous thread (now closed) on this subject, you stated you would feed a low-voltage signal into a transformer and step that up to the 115 V AC required to operate the solenoid in the reed frequency meter. So far, so good. A 6.3 V center-tapped filament transformer with a 115 V AC primary should do nicely. Although most of these are designed for 50 Hz or 60 Hz primary excitation, they will have enough iron to serve at 350 to 450 Hz.

If you use a 555 to make a variable frequency oscillator, you should use the output to clock a D-type flip-flop to create complementary square wave output at half the 555 frequency on the Q and /Q outputs of the flip-flop. So the 555 would oscillate between 700 and 900 Hz. Use the two flip-flop output signals to drive a pair of transistors (2N7000 MOSFETs are easy, but NPN power transistors will also work) connected as switches between the two secondary terminals of the transformer and circuit common. Connect the center tap of the secondary to a +5 V DC supply. You should insert a current-limiting resistor in series between the center-tap and the +5 V DC supply. Use diodes across the transistor switches to protect them from "flyback" voltage generated by the on/off switching. The square waves from the flip-flop will alternately turn the transistors on and produce a high voltage in the transformer primary. You can play around with how much current gets switched by appropriate sizing of the current-limiting resistor in series with the center-tap and the +5 V DC supply. You can even try adding a resonating capacitor to the load side (the primary of the filament transformer) to see what effect this has.

This isn't "rocket science" and should be well within your capabilities. BTW, pin-outs on a 741 op-amp were shown in your first post on this thread. Most folks would just Google "741 datasheet" to get pin-outs. The pin-out sometimes varies with the particular package, which you failed to specify. Do you need help building a 555 variable-frequency oscillator or choosing a flip-flop?

[Harald Kapp] Edit: deleted repeat quotes.

Last edited by a moderator:

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,476
@colum : What's the use of citing Hop repeatedly without adding any comment of your own?

#### colum

Jul 25, 2013
174
@colum : What's the use of citing Hop repeatedly without adding any comment of your own?

I am totally confused about responding to responses in this forum and I would appreciate it if someone would let me know if theres a guide somewhere and also if theres a Free Stuff Forum that I can give stuff away.
I'm not criticizing Electronics Point its just that I'm getting old and not too familiar with stuff that comes easily to a Ten year old. ...Thanks...Jeff

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,476
am totally confused about responding to responses in this forum
When you want to reply to a post, you simply enter the reply in the textbox at the bottom of he page and hit 'post reply'.
If you want to quote some text, mark the text to be quoted and a little button will show in the lower right corner of the marked text. Hit that button and the marked text will be inserted as a quote as in this post. Ten add your remarks.
Note that this type of quoting should be used sparingly, as the text is already there in a previous post. It is often sufficient to refer to this previous post as for example in: ' as Jeff wrote in post #5...' The number of the post is shown in the lower right corner of each post.

Last not least: If you want to give away stuff, there's the 'wanted/for sale' on electronicspoint.

I'm getting old and not too familiar with stuff that comes easily to a Ten year old
Don't worry, we're a mixed crowd with not few old goats active here on this forum . Age is not a reason not to be active here.

#### colum

Jul 25, 2013
174
• Thank you for the advice Harold ....Jeff

Replies
24
Views
640
Replies
6
Views
605
Replies
8
Views
618
Replies
20
Views
3K
Replies
6
Views
638