Help on making LED self check in sequence/for a led tachometer

[dudeon]

hello everyone ,,

ive made an led tachometer using lm3917 and lm3914 ..and its working fine.

so i want to add a startup sequence to the led's( when the circuit is powered ON the Led's must power on in sequence and power off in sequence)..without microcontrollers if possible.


can you please give me some ideas/links to similar projects??

thank you.

 

[alec_t]

I can't find a datasheet for the LM3917. Do you have a link to it? Alternatively, can you confirm its pinout (particularly pin4) is similar to the LM2917 (see attached)?

Can you also post the schematic of your present tacho?

 

[schmitt trigger]

1) you need a one shot (555) to time the startup sequence
2) you also need a DPDT analog switch (4053) driven from the one shot to select the startup sequence. The output of the 4053 would go the 3914 input
3) finally you would require a trianglewave generator....there are dozens of ways to do this, mostly using opamps, google them.

Upon power up, the 555 would enable the 4053 to switch to the triangle generator. Then it would time out and it would revert back to the 3917 for as long as the circuit is powered up..

 

[dudeon]

here is the datasheet..

https://www.electro-tech-online.com/custompdfs/2013/08/lm3914.pdf

this is the tachometer schematic
**broken link removed**

 

[dudeon]

thank you so much ,,very useful..will look...did not know the correct terms to search

 

[dudeon]

ive replied with the link to datasheet (waiting for moderator approval).. and im using the 14pin version of 2917..

 

[dudeon]

1) you need a one shot (555) to time the startup sequence
2) you also need a DPDT analog switch (4053) driven from the one shot to select the startup sequence. The output of the 4053 would go the 3914 input
3) finally you would require a trianglewave generator....there are dozens of ways to do this, mostly using opamps, google them.

Upon power up, the 555 would enable the 4053 to switch to the triangle generator. Then it would time out and it would revert back to the 3917 for as long as the circuit is powered up..

so One "/\" of triangular wave is the time for 555 right ..is this what you mean ??

 

[schmitt trigger]

Correct!

 

[alec_t]

Here's a suggested circuit:

It ramps up over ~1.5sec to 2.5V (full-scale for your 2 x LM3914), then ramps down over ~1.5sec to ~0.1V, then outputs the tacho voltage.

 

[dudeon]

Here's a suggested circuit:
View attachment 75250
It ramps up over ~1.5sec to 2.5V (full-scale for your 2 x LM3914), then ramps down over ~1.5sec to ~0.1V, then outputs the tacho voltage.

actually ive modified the full scale to be 4.5v.. so what should i change in your ckt ?

 

[alec_t]

Here's a revised circuit to cater for full-scale volts up to ~5V by adjustment of the pot.


Edit: C1 should be connected to the SET inputs of U1a/U1b via a 330k resistor (not shown).

 

[dudeon]

Here's a revised circuit to cater for full-scale volts up to ~5V by adjustment of the pot.
View attachment 75257

Edit: C1 should be connected to the SET inputs of U1a/U1b via a 330k resistor (not shown).

im very thank full for your help but im not familiar with these components and ive searched enough(lost my head)..im not sure of the working and how it produces a triangle wave ..

any resources ??

 

[alec_t]

im not sure of the working and how it produces a triangle wave ..

At power on C1 provides a brief pulse to the SET inputs of both D-latches in the CD4013 IC. The Q pins of both latches go high and capacitors C2, C3 charge up via R3, R5 respectively. C3 charges more slowly than C2. R4/R6 form a potential divider. When the output voltage from this reaches ~ 4.5V U1a resets and its Q pin goes low. C2 now discharges via R3 and also via R2/D1 (which are there to speed up the discharge, but are optional if you don't mind the discharge time being ~ twice the charge time). It is the rising and falling voltage on C2 which is the 'triangle wave'. It's not a true triangle as the charge/discharge is exponential rather than linear, but is good enough for testing the LEDs. By the time C2 has discharged to ~0.2V C3 has charged to 4.5V and causes U1b to reset. While U1b is in the 'set' state the C2 voltage, tapped off as required by the pot, is fed via gate U2a to the LM3914. Once U1b has reset the tacho voltage instead is fed via gate U2b to the LM3914.
Here's what the circuit output would look like: